diff options
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/Locking | 246 | ||||
| -rw-r--r-- | Documentation/filesystems/dentry-locking.txt | 174 | ||||
| -rw-r--r-- | Documentation/filesystems/path-lookup.txt | 382 | ||||
| -rw-r--r-- | Documentation/filesystems/porting | 69 | ||||
| -rw-r--r-- | Documentation/filesystems/proc.txt | 24 | ||||
| -rw-r--r-- | Documentation/filesystems/vfs.txt | 90 |
6 files changed, 663 insertions, 322 deletions
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking index a91f30890011..977d8919cc69 100644 --- a/Documentation/filesystems/Locking +++ b/Documentation/filesystems/Locking @@ -9,23 +9,25 @@ be able to use diff(1). --------------------------- dentry_operations -------------------------- prototypes: - int (*d_revalidate)(struct dentry *, int); - int (*d_hash) (struct dentry *, struct qstr *); - int (*d_compare) (struct dentry *, struct qstr *, struct qstr *); + int (*d_revalidate)(struct dentry *, struct nameidata *); + int (*d_hash)(const struct dentry *, const struct inode *, + struct qstr *); + int (*d_compare)(const struct dentry *, const struct inode *, + const struct dentry *, const struct inode *, + unsigned int, const char *, const struct qstr *); int (*d_delete)(struct dentry *); void (*d_release)(struct dentry *); void (*d_iput)(struct dentry *, struct inode *); char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen); locking rules: - none have BKL - dcache_lock rename_lock ->d_lock may block -d_revalidate: no no no yes -d_hash no no no yes -d_compare: no yes no no -d_delete: yes no yes no -d_release: no no no yes -d_iput: no no no yes + rename_lock ->d_lock may block rcu-walk +d_revalidate: no no yes (ref-walk) maybe +d_hash no no no maybe +d_compare: yes no no maybe +d_delete: no yes no no +d_release: no no yes no +d_iput: no no yes no d_dname: no no no no --------------------------- inode_operations --------------------------- @@ -42,18 +44,23 @@ ata *); int (*rename) (struct inode *, struct dentry *, struct inode *, struct dentry *); int (*readlink) (struct dentry *, char __user *,int); - int (*follow_link) (struct dentry *, struct nameidata *); + void * (*follow_link) (struct dentry *, struct nameidata *); + void (*put_link) (struct dentry *, struct nameidata *, void *); void (*truncate) (struct inode *); - int (*permission) (struct inode *, int, struct nameidata *); + int (*permission) (struct inode *, int, unsigned int); + int (*check_acl)(struct inode *, int, unsigned int); int (*setattr) (struct dentry *, struct iattr *); int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *); int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); ssize_t (*listxattr) (struct dentry *, char *, size_t); int (*removexattr) (struct dentry *, const char *); + void (*truncate_range)(struct inode *, loff_t, loff_t); + long (*fallocate)(struct inode *inode, int mode, loff_t offset, loff_t len); + int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len); locking rules: - all may block, none have BKL + all may block i_mutex(inode) lookup: yes create: yes @@ -66,19 +73,24 @@ rmdir: yes (both) (see below) rename: yes (all) (see below) readlink: no follow_link: no +put_link: no truncate: yes (see below) setattr: yes -permission: no +permission: no (may not block if called in rcu-walk mode) +check_acl: no getattr: no setxattr: yes getxattr: no listxattr: no removexattr: yes +truncate_range: yes +fallocate: no +fiemap: no Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on victim. cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem. ->truncate() is never called directly - it's a callback, not a -method. It's called by vmtruncate() - library function normally used by +method. It's called by vmtruncate() - deprecated library function used by ->setattr(). Locking information above applies to that call (i.e. is inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been passed). @@ -91,7 +103,7 @@ prototypes: struct inode *(*alloc_inode)(struct super_block *sb); void (*destroy_inode)(struct inode *); void (*dirty_inode) (struct inode *); - int (*write_inode) (struct inode *, int); + int (*write_inode) (struct inode *, struct writeback_control *wbc); int (*drop_inode) (struct inode *); void (*evict_inode) (struct inode *); void (*put_super) (struct super_block *); @@ -105,10 +117,10 @@ prototypes: int (*show_options)(struct seq_file *, struct vfsmount *); ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); + int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t); locking rules: All may block [not true, see below] - None have BKL s_umount alloc_inode: destroy_inode: @@ -127,6 +139,7 @@ umount_begin: no show_options: no (namespace_sem) quota_read: no (see below) quota_write: no (see below) +bdev_try_to_free_page: no (see below) ->statfs() has s_umount (shared) when called by ustat(2) (native or compat), but that's an accident of bad API; s_umount is used to pin @@ -139,19 +152,25 @@ be the only ones operating on the quota file by the quota code (via dqio_sem) (unless an admin really wants to screw up something and writes to quota files with quotas on). For other details about locking see also dquot_operations section. +->bdev_try_to_free_page is called from the ->releasepage handler of +the block device inode. See there for more details. --------------------------- file_system_type --------------------------- prototypes: int (*get_sb) (struct file_system_type *, int, const char *, void *, struct vfsmount *); + struct dentry *(*mount) (struct file_system_type *, int, + const char *, void *); void (*kill_sb) (struct super_block *); locking rules: - may block BKL -get_sb yes no -kill_sb yes no + may block +get_sb yes +mount yes +kill_sb yes ->get_sb() returns error or 0 with locked superblock attached to the vfsmount (exclusive on ->s_umount). +->mount() returns ERR_PTR or the root dentry. ->kill_sb() takes a write-locked superblock, does all shutdown work on it, unlocks and drops the reference. @@ -173,28 +192,38 @@ prototypes: sector_t (*bmap)(struct address_space *, sector_t); int (*invalidatepage) (struct page *, unsigned long); int (*releasepage) (struct page *, int); + void (*freepage)(struct page *); int (*direct_IO)(int, struct kiocb *, const struct iovec *iov, loff_t offset, unsigned long nr_segs); - int (*launder_page) (struct page *); + int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **, + unsigned long *); + int (*migratepage)(struct address_space *, struct page *, struct page *); + int (*launder_page)(struct page *); + int (*is_partially_uptodate)(struct page *, read_descriptor_t *, unsigned long); + int (*error_remove_page)(struct address_space *, struct page *); locking rules: - All except set_page_dirty may block - - BKL PageLocked(page) i_mutex -writepage: no yes, unlocks (see below) -readpage: no yes, unlocks -sync_page: no maybe -writepages: no -set_page_dirty no no -readpages: no -write_begin: no locks the page yes -write_end: no yes, unlocks yes -perform_write: no n/a yes -bmap: no -invalidatepage: no yes -releasepage: no yes -direct_IO: no -launder_page: no yes + All except set_page_dirty and freepage may block + + PageLocked(page) i_mutex +writepage: yes, unlocks (see below) +readpage: yes, unlocks +sync_page: maybe +writepages: +set_page_dirty no +readpages: +write_begin: locks the page yes +write_end: yes, unlocks yes +bmap: +invalidatepage: yes +releasepage: yes +freepage: yes +direct_IO: +get_xip_mem: maybe +migratepage: yes (both) +launder_page: yes +is_partially_uptodate: yes +error_remove_page: yes ->write_begin(), ->write_end(), ->sync_page() and ->readpage() may be called from the request handler (/dev/loop). @@ -274,9 +303,8 @@ under spinlock (it cannot block) and is sometimes called with the page not locked. ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some -filesystems and by the swapper. The latter will eventually go away. All -instances do not actually need the BKL. Please, keep it that way and don't -breed new callers. +filesystems and by the swapper. The latter will eventually go away. Please, +keep it that way and don't breed new callers. ->invalidatepage() is called when the filesystem must attempt to drop some or all of the buffers from the page when it is being truncated. It @@ -288,53 +316,46 @@ buffers from the page in preparation for freeing it. It returns zero to indicate that the buffers are (or may be) freeable. If ->releasepage is zero, the kernel assumes that the fs has no private interest in the buffers. + ->freepage() is called when the kernel is done dropping the page +from the page cache. + ->launder_page() may be called prior to releasing a page if it is still found to be dirty. It returns zero if the page was successfully cleaned, or an error value if not. Note that in order to prevent the page getting mapped back in and redirtied, it needs to be kept locked across the entire operation. - Note: currently almost all instances of address_space methods are -using BKL for internal serialization and that's one of the worst sources -of contention. Normally they are calling library functions (in fs/buffer.c) -and pass foo_get_block() as a callback (on local block-based filesystems, -indeed). BKL is not needed for library stuff and is usually taken by -foo_get_block(). It's an overkill, since block bitmaps can be protected by -internal fs locking and real critical areas are much smaller than the areas -filesystems protect now. - ----------------------- file_lock_operations ------------------------------ prototypes: - void (*fl_insert)(struct file_lock *); /* lock insertion callback */ - void (*fl_remove)(struct file_lock *); /* lock removal callback */ void (*fl_copy_lock)(struct file_lock *, struct file_lock *); void (*fl_release_private)(struct file_lock *); locking rules: - BKL may block -fl_insert: yes no -fl_remove: yes no -fl_copy_lock: yes no -fl_release_private: yes yes + file_lock_lock may block +fl_copy_lock: yes no +fl_release_private: maybe no ----------------------- lock_manager_operations --------------------------- prototypes: int (*fl_compare_owner)(struct file_lock *, struct file_lock *); void (*fl_notify)(struct file_lock *); /* unblock callback */ + int (*fl_grant)(struct file_lock *, struct file_lock *, int); void (*fl_release_private)(struct file_lock *); void (*fl_break)(struct file_lock *); /* break_lease callback */ + int (*fl_mylease)(struct file_lock *, struct file_lock *); + int (*fl_change)(struct file_lock **, int); locking rules: - BKL may block -fl_compare_owner: yes no -fl_notify: yes no -fl_release_private: yes yes -fl_break: yes no - - Currently only NFSD and NLM provide instances of this class. None of the -them block. If you have out-of-tree instances - please, show up. Locking -in that area will change. + file_lock_lock may block +fl_compare_owner: yes no +fl_notify: yes no +fl_grant: no no +fl_release_private: maybe no +fl_break: yes no +fl_mylease: yes no +fl_change yes no + --------------------------- buffer_head ----------------------------------- prototypes: void (*b_end_io)(struct buffer_head *bh, int uptodate); @@ -359,17 +380,17 @@ prototypes: void (*swap_slot_free_notify) (struct block_device *, unsigned long); locking rules: - BKL bd_mutex -open: no yes -release: no yes -ioctl: no no -compat_ioctl: no no -direct_access: no no -media_changed: no no -unlock_native_capacity: no no -revalidate_disk: no no -getgeo: no no -swap_slot_free_notify: no no (see below) + bd_mutex +open: yes +release: yes +ioctl: no +compat_ioctl: no +direct_access: no +media_changed: no +unlock_native_capacity: no +revalidate_disk: no +getgeo: no +swap_slot_free_notify: no (see below) media_changed, unlock_native_capacity and revalidate_disk are called only from check_disk_change(). @@ -408,34 +429,21 @@ prototypes: unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); int (*check_flags)(int); + int (*flock) (struct file *, int, struct file_lock *); + ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, + size_t, unsigned int); + ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, + size_t, unsigned int); + int (*setlease)(struct file *, long, struct file_lock **); }; locking rules: - All may block. - BKL -llseek: no (see below) -read: no -aio_read: no -write: no -aio_write: no -readdir: no -poll: no -unlocked_ioctl: no -compat_ioctl: no -mmap: no -open: no -flush: no -release: no -fsync: no (see below) -aio_fsync: no -fasync: no -lock: yes -readv: no -writev: no -sendfile: no -sendpage: no -get_unmapped_area: no -check_flags: no + All may block except for ->setlease. + No VFS locks held on entry except for ->fsync and ->setlease. + +->fsync() has i_mutex on inode. + +->setlease has the file_list_lock held and must not sleep. ->llseek() locking has moved from llseek to the individual llseek implementations. If your fs is not using generic_file_llseek, you @@ -445,17 +453,10 @@ mutex or just to use i_size_read() instead. Note: this does not protect the file->f_pos against concurrent modifications since this is something the userspace has to take care about. -Note: ext2_release() was *the* source of contention on fs-intensive -loads and dropping BKL on ->release() helps to get rid of that (we still -grab BKL for cases when we close a file that had been opened r/w, but that -can and should be done using the internal locking with smaller critical areas). -Current worst offender is ext2_get_block()... - -->fasync() is called without BKL protection, and is responsible for -maintaining the FASYNC bit in filp->f_flags. Most instances call -fasync_helper(), which does that maintenance, so it's not normally -something one needs to worry about. Return values > 0 will be mapped to -zero in the VFS layer. +->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags. +Most instances call fasync_helper(), which does that maintenance, so it's +not normally something one needs to worry about. Return values > 0 will be +mapped to zero in the VFS layer. ->readdir() and ->ioctl() on directories must be changed. Ideally we would move ->readdir() to inode_operations and use a separate method for directory @@ -466,8 +467,6 @@ components. And there are other reasons why the current interface is a mess... ->read on directories probably must go away - we should just enforce -EISDIR in sys_read() and friends. -->fsync() has i_mutex on inode. - --------------------------- dquot_operations ------------------------------- prototypes: int (*write_dquot) (struct dquot *); @@ -502,12 +501,12 @@ prototypes: int (*access)(struct vm_area_struct *, unsigned long, void*, int, int); locking rules: - BKL mmap_sem PageLocked(page) -open: no yes -close: no yes -fault: no yes can return with page locked -page_mkwrite: no yes can return with page locked -access: no yes + mmap_sem PageLocked(page) +open: yes +close: yes +fault: yes can return with page locked +page_mkwrite: yes can return with page locked +access: yes ->fault() is called when a previously not present pte is about to be faulted in. The filesystem must find and return the page associated @@ -534,6 +533,3 @@ VM_IO | VM_PFNMAP VMAs. (if you break something or notice that it is broken and do not fix it yourself - at least put it here) - -ipc/shm.c::shm_delete() - may need BKL. -->read() and ->write() in many drivers are (probably) missing BKL. diff --git a/Documentation/filesystems/dentry-locking.txt b/Documentation/filesystems/dentry-locking.txt deleted file mode 100644 index 79334ed5daa7..000000000000 --- a/Documentation/filesystems/dentry-locking.txt +++ /dev/null @@ -1,174 +0,0 @@ -RCU-based dcache locking model -============================== - -On many workloads, the most common operation on dcache is to look up a -dentry, given a parent dentry and the name of the child. Typically, -for every open(), stat() etc., the dentry corresponding to the -pathname will be looked up by walking the tree starting with the first -component of the pathname and using that dentry along with the next -component to look up the next level and so on. Since it is a frequent -operation for workloads like multiuser environments and web servers, -it is important to optimize this path. - -Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus in -every component during path look-up. Since 2.5.10 onwards, fast-walk -algorithm changed this by holding the dcache_lock at the beginning and -walking as many cached path component dentries as possible. This -significantly decreases the number of acquisition of -dcache_lock. However it also increases the lock hold time -significantly and affects performance in large SMP machines. Since -2.5.62 kernel, dcache has been using a new locking model that uses RCU -to make dcache look-up lock-free. - -The current dcache locking model is not very different from the -existing dcache locking model. Prior to 2.5.62 kernel, dcache_lock -protected the hash chain, d_child, d_alias, d_lru lists as well as -d_inode and several other things like mount look-up. RCU-based changes -affect only the way the hash chain is protected. For everything else -the dcache_lock must be taken for both traversing as well as -updating. The hash chain updates too take the dcache_lock. The -significant change is the way d_lookup traverses the hash chain, it -doesn't acquire the dcache_lock for this and rely on RCU to ensure -that the dentry has not been *freed*. - - -Dcache locking details -====================== - -For many multi-user workloads, open() and stat() on files are very -frequently occurring operations. Both involve walking of path names to -find the dentry corresponding to the concerned file. In 2.4 kernel, -dcache_lock was held during look-up of each path component. Contention -and cache-line bouncing of this global lock caused significant -scalability problems. With the introduction of RCU in Linux kernel, -this was worked around by making the look-up of path components during -path walking lock-free. - - -Safe lock-free look-up of dcache hash table -=========================================== - -Dcache is a complex data structure with the hash table entries also -linked together in other lists. In 2.4 kernel, dcache_lock protected -all the lists. We applied RCU only on hash chain walking. The rest of -the lists are still protected by dcache_lock. Some of the important -changes are : - -1. The deletion from hash chain is done using hlist_del_rcu() macro - which doesn't initialize next pointer of the deleted dentry and - this allows us to walk safely lock-free while a deletion is - happening. - -2. Insertion of a dentry into the hash table is done using - hlist_add_head_rcu() which take care of ordering the writes - the - writes to the dentry must be visible before the dentry is - inserted. This works in conjunction with hlist_for_each_rcu(), - which has since been replaced by hlist_for_each_entry_rcu(), while - walking the hash chain. The only requirement is that all - initialization to the dentry must be done before - hlist_add_head_rcu() since we don't have dcache_lock protection - while traversing the hash chain. This isn't different from the - existing code. - -3. The dentry looked up without holding dcache_lock by cannot be - returned for walking if it is unhashed. It then may have a NULL - d_inode or other bogosity since RCU doesn't protect the other - fields in the dentry. We therefore use a flag DCACHE_UNHASHED to - indicate unhashed dentries and use this in conjunction with a - per-dentry lock (d_lock). Once looked up without the dcache_lock, - we acquire the per-dentry lock (d_lock) and check if the dentry is - unhashed. If so, the look-up is failed. If not, the reference count - of the dentry is increased and the dentry is returned. - -4. Once a dentry is looked up, it must be ensured during the path walk - for that component it doesn't go away. In pre-2.5.10 code, this was - done holding a reference to the dentry. dcache_rcu does the same. - In some sense, dcache_rcu path walking looks like the pre-2.5.10 - version. - -5. All dentry hash chain updates must take the dcache_lock as well as - the per-dentry lock in that order. dput() does this to ensure that - a dentry that has just been looked up in another CPU doesn't get - deleted before dget() can be done on it. - -6. There are several ways to do reference counting of RCU protected - objects. One such example is in ipv4 route cache where deferred - freeing (using call_rcu()) is done as soon as the reference count - goes to zero. This cannot be done in the case of dentries because - tearing down of dentries require blocking (dentry_iput()) which - isn't supported from RCU callbacks. Instead, tearing down of - dentries happen synchronously in dput(), but actual freeing happens - later when RCU grace period is over. This allows safe lock-free - walking of the hash chains, but a matched dentry may have been - partially torn down. The checking of DCACHE_UNHASHED flag with - d_lock held detects such dentries and prevents them from being - returned from look-up. - - -Maintaining POSIX rename semantics -================================== - -Since look-up of dentries is lock-free, it can race against a -concurrent rename operation. For example, during rename of file A to -B, look-up of either A or B must succeed. So, if look-up of B happens -after A has been removed from the hash chain but not added to the new -hash chain, it may fail. Also, a comparison while the name is being -written concurrently by a rename may result in false positive matches -violating rename semantics. Issues related to race with rename are -handled as described below : - -1. Look-up can be done in two ways - d_lookup() which is safe from - simultaneous renames and __d_lookup() which is not. If - __d_lookup() fails, it must be followed up by a d_lookup() to - correctly determine whether a dentry is in the hash table or - not. d_lookup() protects look-ups using a sequence lock - (rename_lock). - -2. The name associated with a dentry (d_name) may be changed if a - rename is allowed to happen simultaneously. To avoid memcmp() in - __d_lookup() go out of bounds due to a rename and false positive - comparison, the name comparison is done while holding the - per-dentry lock. This prevents concurrent renames during this - operation. - -3. Hash table walking during look-up may move to a different bucket as - the current dentry is moved to a different bucket due to rename. - But we use hlists in dcache hash table and they are - null-terminated. So, even if a dentry moves to a different bucket, - hash chain walk will terminate. [with a list_head list, it may not - since termination is when the list_head in the original bucket is - reached]. Since we redo the d_parent check and compare name while - holding d_lock, lock-free look-up will not race against d_move(). - -4. There can be a theoretical race when a dentry keeps coming back to - original bucket due to double moves. Due to this look-up may - consider that it has never moved and can end up in a infinite loop. - But this is not any worse that theoretical livelocks we already - have in the kernel. - - -Important guidelines for filesystem developers related to dcache_rcu -==================================================================== - -1. Existing dcache interfaces (pre-2.5.62) exported to filesystem - don't change. Only dcache internal implementation changes. However - filesystems *must not* delete from the dentry hash chains directly - using the list macros like allowed earlier. They must use dcache - APIs like d_drop() or __d_drop() depending on the situation. - -2. d_flags is now protected by a per-dentry lock (d_lock). All access - to d_flags must be protected by it. - -3. For a hashed dentry, checking of d_count needs to be protected by - d_lock. - - -Papers and other documentation on dcache locking -================================================ - -1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124). - -2. http://lse.sourceforge.net/locking/dcache/dcache.html - - - diff --git a/Documentation/filesystems/path-lookup.txt b/Documentation/filesystems/path-lookup.txt new file mode 100644 index 000000000000..eb59c8b44be9 --- /dev/null +++ b/Documentation/filesystems/path-lookup.txt @@ -0,0 +1,382 @@ +Path walking and name lookup locking +==================================== + +Path resolution is the finding a dentry corresponding to a path name string, by +performing a path walk. Typically, for every open(), stat() etc., the path name +will be resolved. Paths are resolved by walking the namespace tree, starting +with the first component of the pathname (eg. root or cwd) with a known dentry, +then finding the child of that dentry, which is named the next component in the +path string. Then repeating the lookup from the child dentry and finding its +child with the next element, and so on. + +Since it is a frequent operation for workloads like multiuser environments and +web servers, it is important to optimize this code. + +Path walking synchronisation history: +Prior to 2.5.10, dcache_lock was acquired in d_lookup (dcache hash lookup) and +thus in every component during path look-up. Since 2.5.10 onwards, fast-walk +algorithm changed this by holding the dcache_lock at the beginning and walking +as many cached path component dentries as possible. This significantly +decreases the number of acquisition of dcache_lock. However it also increases +the lock hold time significantly and affects performance in large SMP machines. +Since 2.5.62 kernel, dcache has been using a new locking model that uses RCU to +make dcache look-up lock-free. + +All the above algorithms required taking a lock and reference count on the +dentry that was looked up, so that may be used as the basis for walking the +next path element. This is inefficient and unscalable. It is inefficient +because of the locks and atomic operations required for every dentry element +slows things down. It is not scalable because many parallel applications that +are path-walk intensive tend to do path lookups starting from a common dentry +(usually, the root "/" or current working directory). So contention on these +common path elements causes lock and cacheline queueing. + +Since 2.6.38, RCU is used to make a significant part of the entire path walk +(including dcache look-up) completely "store-free" (so, no locks, atomics, or +even stores into cachelines of common dentries). This is known as "rcu-walk" +path walking. + +Path walking overview +===================== + +A name string specifies a start (root directory, cwd, fd-relative) and a +sequence of elements (directory entry names), which together refer to a path in +the namespace. A path is represented as a (dentry, vfsmount) tuple. The name +elements are sub-strings, seperated by '/'. + +Name lookups will want to find a particular path that a name string refers to +(usually the final element, or parent of final element). This is done by taking +the path given by the name's starting point (which we know in advance -- eg. +current->fs->cwd or current->fs->root) as the first parent of the lookup. Then +iteratively for each subsequent name element, look up the child of the current +parent with the given name and if it is not the desired entry, make it the +parent for the next lookup. + +A parent, of course, must be a directory, and we must have appropriate +permissions on the parent inode to be able to walk into it. + +Turning the child into a parent for the next lookup requires more checks and +procedures. Symlinks essentially substitute the symlink name for the target +name in the name string, and require some recursive path walking. Mount points +must be followed into (thus changing the vfsmount that subsequent path elements +refer to), switching from the mount point path to the root of the particular +mounted vfsmount. These behaviours are variously modified depending on the +exact path walking flags. + +Path walking then must, broadly, do several particular things: +- find the start point of the walk; +- perform permissions and validity checks on inodes; +- perform dcache hash name lookups on (parent, name element) tuples; +- traverse mount points; +- traverse symlinks; +- lookup and create missing parts of the path on demand. + +Safe store-free look-up of dcache hash table +============================================ + +Dcache name lookup +------------------ +In order to lookup a dcache (parent, name) tuple, we take a hash on the tuple +and use that to select a bucket in the dcache-hash table. The list of entries +in that bucket is then walked, and we do a full comparison of each entry +against our (parent, name) tuple. + +The hash lists are RCU protected, so list walking is not serialised with +concurrent updates (insertion, deletion from the hash). This is a standard RCU +list application with the exception of renames, which will be covered below. + +Parent and name members of a dentry, as well as its membership in the dcache +hash, and its inode are protected by the per-dentry d_lock spinlock. A +reference is taken on the dentry (while the fields are verified under d_lock), +and this stabilises its d_inode pointer and actual inode. This gives a stable +point to perform the next step of our path walk against. + +These members are also protected by d_seq seqlock, although this offers +read-only protection and no durability of results, so care must be taken when +using d_seq for synchronisation (see seqcount based lookups, below). + +Renames +------- +Back to the rename case. In usual RCU protected lists, the only operations that +will happen to an object is insertion, and then eventually removal from the +list. The object will not be reused until an RCU grace period is complete. +This ensures the RCU list traversal primitives can run over the object without +problems (see RCU documentation for how this works). + +However when a dentry is renamed, its hash value can change, requiring it to be +moved to a new hash list. Allocating and inserting a new alias would be +expensive and also problematic for directory dentries. Latency would be far to +high to wait for a grace period after removing the dentry and before inserting +it in the new hash bucket. So what is done is to insert the dentry into the +new list immediately. + +However, when the dentry's list pointers are updated to point to objects in the +new list before waiting for a grace period, this can result in a concurrent RCU +lookup of the old list veering off into the new (incorrect) list and missing +the remaining dentries on the list. + +There is no fundamental problem with walking down the wrong list, because the +dentry comparisons will never match. However it is fatal to miss a matching +dentry. So a seqlock is used to detect when a rename has occurred, and so the +lookup can be retried. + + 1 2 3 + +---+ +---+ +---+ +hlist-->| N-+->| N-+->| N-+-> +head <--+-P |<-+-P |<-+-P | + +---+ +---+ +---+ + +Rename of dentry 2 may require it deleted from the above list, and inserted +into a new list. Deleting 2 gives the following list. + + 1 3 + +---+ +---+ (don't worry, the longer pointers do not +hlist-->| N-+-------->| N-+-> impose a measurable performance overhead +head <--+-P |<--------+-P | on modern CPUs) + +---+ +---+ + ^ 2 ^ + | +---+ | + | | N-+----+ + +----+-P | + +---+ + +This is a standard RCU-list deletion, which leaves the deleted object's +pointers intact, so a concurrent list walker that is currently looking at +object 2 will correctly continue to object 3 when it is time to traverse the +next object. + +However, when inserting object 2 onto a new list, we end up with this: + + 1 3 + +---+ +---+ +hlist-->| N-+-------->| N-+-> +head <--+-P |<--------+-P | + +---+ +---+ + 2 + +---+ + | N-+----> + <----+-P | + +---+ + +Because we didn't wait for a grace period, there may be a concurrent lookup +still at 2. Now when it follows 2's 'next' pointer, it will walk off into +another list without ever having checked object 3. + +A related, but distinctly different, issue is that of rename atomicity versus +lookup operations. If a file is renamed from 'A' to 'B', a lookup must only +find either 'A' or 'B'. So if a lookup of 'A' returns NULL, a subsequent lookup +of 'B' must succeed (note the reverse is not true). + +Between deleting the dentry from the old hash list, and inserting it on the new +hash list, a lookup may find neither 'A' nor 'B' matching the dentry. The same +rename seqlock is also used to cover this race in much the same way, by +retrying a negative lookup result if a rename was in progress. + +Seqcount based lookups +---------------------- +In refcount based dcache lookups, d_lock is used to serialise access to +the dentry, stabilising it while comparing its name and parent and then +taking a reference count (the reference count then gives a stable place to +start the next part of the path walk from). + +As explained above, we would like to do path walking without taking locks or +reference counts on intermediate dentries along the path. To do this, a per +dentry seqlock (d_seq) is used to take a "coherent snapshot" of what the dentry +looks like (its name, parent, and inode). That snapshot is then used to start +the next part of the path walk. When loading the coherent snapshot under d_seq, +care must be taken to load the members up-front, and use those pointers rather +than reloading from the dentry later on (otherwise we'd have interesting things +like d_inode going NULL underneath us, if the name was unlinked). + +Also important is to avoid performing any destructive operations (pretty much: +no non-atomic stores to shared data), and to recheck the seqcount when we are +"done" with the operation. Retry or abort if the seqcount does not match. +Avoiding destructive or changing operations means we can easily unwind from +failure. + +What this means is that a caller, provided they are holding RCU lock to +protect the dentry object from disappearing, can perform a seqcount based +lookup which does not increment the refcount on the dentry or write to +it in any way. This returned dentry can be used for subsequent operations, +provided that d_seq is rechecked after that operation is complete. + +Inodes are also rcu freed, so the seqcount lookup dentry's inode may also be +queried for permissions. + +With this two parts of the puzzle, we can do path lookups without taking +locks or refcounts on dentry elements. + +RCU-walk path walking design +============================ + +Path walking code now has two distinct modes, ref-walk and rcu-walk. ref-walk +is the traditional[*] way of performing dcache lookups using d_lock to +serialise concurrent modifications to the dentry and take a reference count on +it. ref-walk is simple and obvious, and may sleep, take locks, etc while path +walking is operating on each dentry. rcu-walk uses seqcount based dentry +lookups, and can perform lookup of intermediate elements without any stores to +shared data in the dentry or inode. rcu-walk can not be applied to all cases, +eg. if the filesystem must sleep or perform non trivial operations, rcu-walk +must be switched to ref-walk mode. + +[*] RCU is still used for the dentry hash lookup in ref-walk, but not the full + path walk. + +Where ref-walk uses a stable, refcounted ``parent'' to walk the remaining +path string, rcu-walk uses a d_seq protected snapshot. When looking up a +child of this parent snapshot, we open d_seq critical section on the child +before closing d_seq critical section on the parent. This gives an interlocking +ladder of snapshots to walk down. + + + proc 101 + /----------------\ + / comm: "vi" \ + / fs.root: dentry0 \ + \ fs.cwd: dentry2 / + \ / + \----------------/ + +So when vi wants to open("/home/npiggin/test.c", O_RDWR), then it will +start from current->fs->root, which is a pinned dentry. Alternatively, +"./test.c" would start from cwd; both names refer to the same path in +the context of proc101. + + dentry 0 + +---------------------+ rcu-walk begins here, we note d_seq, check the + | name: "/" | inode's permission, and then look up the next + | inode: 10 | path element which is "home"... + | children:"home", ...| + +---------------------+ + | + dentry 1 V + +---------------------+ ... which brings us here. We find dentry1 via + | name: "home" | hash lookup, then note d_seq and compare name + | inode: 678 | string and parent pointer. When we have a match, + | children:"npiggin" | we now recheck the d_seq of dentry0. Then we + +---------------------+ check inode and look up the next element. + | + dentry2 V + +---------------------+ Note: if dentry0 is now modified, lookup is + | name: "npiggin" | not necessarily invalid, so we need only keep a + | inode: 543 | parent for d_seq verification, and grandparents + | children:"a.c", ... | can be forgotten. + +---------------------+ + | + dentry3 V + +---------------------+ At this point we have our destination dentry. + | name: "a.c" | We now take its d_lock, verify d_seq of this + | inode: 14221 | dentry. If that checks out, we can increment + | children:NULL | its refcount because we're holding d_lock. + +---------------------+ + +Taking a refcount on a dentry from rcu-walk mode, by taking its d_lock, +re-checking its d_seq, and then incrementing its refcount is called +"dropping rcu" or dropping from rcu-walk into ref-walk mode. + +It is, in some sense, a bit of a house of cards. If the seqcount check of the +parent snapshot fails, the house comes down, because we had closed the d_seq +section on the grandparent, so we have nothing left to stand on. In that case, +the path walk must be fully restarted (which we do in ref-walk mode, to avoid +live locks). It is costly to have a full restart, but fortunately they are +quite rare. + +When we reach a point where sleeping is required, or a filesystem callout +requires ref-walk, then instead of restarting the walk, we attempt to drop rcu +at the last known good dentry we have. Avoiding a full restart in ref-walk in +these cases is fundamental for performance and scalability because blocking +operations such as creates and unlinks are not uncommon. + +The detailed design for rcu-walk is like this: +* LOOKUP_RCU is set in nd->flags, which distinguishes rcu-walk from ref-walk. +* Take the RCU lock for the entire path walk, starting with the acquiring + of the starting path (eg. root/cwd/fd-path). So now dentry refcounts are + not required for dentry persistence. +* synchronize_rcu is called when unregistering a filesystem, so we can + access d_ops and i_ops during rcu-walk. +* Similarly take the vfsmount lock for the entire path walk. So now mnt + refcounts are not required for persistence. Also we are free to perform mount + lookups, and to assume dentry mount points and mount roots are stable up and + down the path. +* Have a per-dentry seqlock to protect the dentry name, parent, and inode, + so we can load this tuple atomically, and also check whether any of its + members have changed. +* Dentry lookups (based on parent, candidate string tuple) recheck the parent + sequence after the child is found in case anything changed in the parent + during the path walk. +* inode is also RCU protected so we can load d_inode and use the inode for + limited things. +* i_mode, i_uid, i_gid can be tested for exec permissions during path walk. +* i_op can be loaded. +* When the destination dentry is reached, drop rcu there (ie. take d_lock, + verify d_seq, increment refcount). +* If seqlock verification fails anywhere along the path, do a full restart + of the path lookup in ref-walk mode. -ECHILD tends to be used (for want of + a better errno) to signal an rcu-walk failure. + +The cases where rcu-walk cannot continue are: +* NULL dentry (ie. any uncached path element) +* Following links + +It may be possible eventually to make following links rcu-walk aware. + +Uncached path elements will always require dropping to ref-walk mode, at the +very least because i_mutex needs to be grabbed, and objects allocated. + +Final note: +"store-free" path walking is not strictly store free. We take vfsmount lock +and refcounts (both of which can be made per-cpu), and we also store to the +stack (which is essentially CPU-local), and we also have to take locks and +refcount on final dentry. + +The point is that shared data, where practically possible, is not locked +or stored into. The result is massive improvements in performance and +scalability of path resolution. + + +Interesting statistics +====================== + +The following table gives rcu lookup statistics for a few simple workloads +(2s12c24t Westmere, debian non-graphical system). Ungraceful are attempts to +drop rcu that fail due to d_seq failure and requiring the entire path lookup +again. Other cases are successful rcu-drops that are required before the final +element, nodentry for missing dentry, revalidate for filesystem revalidate +routine requiring rcu drop, permission for permission check requiring drop, +and link for symlink traversal requiring drop. + + rcu-lookups restart nodentry link revalidate permission +bootup 47121 0 4624 1010 10283 7852 +dbench 25386793 0 6778659(26.7%) 55 549 1156 +kbuild 2696672 10 64442(2.3%) 108764(4.0%) 1 1590 +git diff 39605 0 28 2 0 106 +vfstest 24185492 4945 708725(2.9%) 1076136(4.4%) 0 2651 + +What this shows is that failed rcu-walk lookups, ie. ones that are restarted +entirely with ref-walk, are quite rare. Even the "vfstest" case which +specifically has concurrent renames/mkdir/rmdir/ creat/unlink/etc to excercise +such races is not showing a huge amount of restarts. + +Dropping from rcu-walk to ref-walk mean that we have encountered a dentry where +the reference count needs to be taken for some reason. This is either because +we have reached the target of the path walk, or because we have encountered a +condition that can't be resolved in rcu-walk mode. Ideally, we drop rcu-walk +only when we have reached the target dentry, so the other statistics show where +this does not happen. + +Note that a graceful drop from rcu-walk mode due to something such as the +dentry not existing (which can be common) is not necessarily a failure of +rcu-walk scheme, because some elements of the path may have been walked in +rcu-walk mode. The further we get from common path elements (such as cwd or +root), the less contended the dentry is likely to be. The closer we are to +common path elements, the more likely they will exist in dentry cache. + + +Papers and other documentation on dcache locking +================================================ + +1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124). + +2. http://lse.sourceforge.net/locking/dcache/dcache.html + + diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting index b12c89538680..07a32b42cf9c 100644 --- a/Documentation/filesystems/porting +++ b/Documentation/filesystems/porting @@ -216,7 +216,6 @@ had ->revalidate()) add calls in ->follow_link()/->readlink(). ->d_parent changes are not protected by BKL anymore. Read access is safe if at least one of the following is true: * filesystem has no cross-directory rename() - * dcache_lock is held * we know that parent had been locked (e.g. we are looking at ->d_parent of ->lookup() argument). * we are called from ->rename(). @@ -318,3 +317,71 @@ if it's zero is not *and* *never* *had* *been* enough. Final unlink() and iput( may happen while the inode is in the middle of ->write_inode(); e.g. if you blindly free the on-disk inode, you may end up doing that while ->write_inode() is writing to it. + +--- +[mandatory] + + .d_delete() now only advises the dcache as to whether or not to cache +unreferenced dentries, and is now only called when the dentry refcount goes to +0. Even on 0 refcount transition, it must be able to tolerate being called 0, +1, or more times (eg. constant, idempotent). + +--- +[mandatory] + + .d_compare() calling convention and locking rules are significantly +changed. Read updated documentation in Documentation/filesystems/vfs.txt (and +look at examples of other filesystems) for guidance. + +--- +[mandatory] + + .d_hash() calling convention and locking rules are significantly +changed. Read updated documentation in Documentation/filesystems/vfs.txt (and +look at examples of other filesystems) for guidance. + +--- +[mandatory] + dcache_lock is gone, replaced by fine grained locks. See fs/dcache.c +for details of what locks to replace dcache_lock with in order to protect +particular things. Most of the time, a filesystem only needs ->d_lock, which +protects *all* the dcache state of a given dentry. + +-- +[mandatory] + + Filesystems must RCU-free their inodes, if they can have been accessed +via rcu-walk path walk (basically, if the file can have had a path name in the +vfs namespace). + + i_dentry and i_rcu share storage in a union, and the vfs expects +i_dentry to be reinitialized before it is freed, so an: + + INIT_LIST_HEAD(&inode->i_dentry); + +must be done in the RCU callback. + +-- +[recommended] + vfs now tries to do path walking in "rcu-walk mode", which avoids +atomic operations and scalability hazards on dentries and inodes (see +Documentation/filesystems/path-walk.txt). d_hash and d_compare changes (above) +are examples of the changes required to support this. For more complex +filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so +no changes are required to the filesystem. However, this is costly and loses +the benefits of rcu-walk mode. We will begin to add filesystem callbacks that +are rcu-walk aware, shown below. Filesystems should take advantage of this +where possible. + +-- +[mandatory] + d_revalidate is a callback that is made on every path element (if +the filesystem provides it), which requires dropping out of rcu-walk mode. This +may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be +returned if the filesystem cannot handle rcu-walk. See +Documentation/filesystems/vfs.txt for more details. + + permission and check_acl are inode permission checks that are called +on many or all directory inodes on the way down a path walk (to check for +exec permission). These must now be rcu-walk aware (flags & IPERM_RCU). See +Documentation/filesystems/vfs.txt for more details. diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt index e73df2722ff3..9471225212c4 100644 --- a/Documentation/filesystems/proc.txt +++ b/Documentation/filesystems/proc.txt @@ -1181,6 +1181,30 @@ Table 1-12: Files in /proc/fs/ext4/<devname> mb_groups details of multiblock allocator buddy cache of free blocks .............................................................................. +2.0 /proc/consoles +------------------ +Shows registered system console lines. + +To see which character device lines are currently used for the system console +/dev/console, you may simply look into the file /proc/consoles: + + > cat /proc/consoles + tty0 -WU (ECp) 4:7 + ttyS0 -W- (Ep) 4:64 + +The columns are: + + device name of the device + operations R = can do read operations + W = can do write operations + U = can do unblank + flags E = it is enabled + C = it is prefered console + B = it is primary boot console + p = it is used for printk buffer + b = it is not a TTY but a Braille device + a = it is safe to use when cpu is offline + major:minor major and minor number of the device separated by a colon ------------------------------------------------------------------------------ Summary diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index ed7e5efc06d8..fbb324e2bd43 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -325,7 +325,8 @@ struct inode_operations { void * (*follow_link) (struct dentry *, struct nameidata *); void (*put_link) (struct dentry *, struct nameidata *, void *); void (*truncate) (struct inode *); - int (*permission) (struct inode *, int, struct nameidata *); + int (*permission) (struct inode *, int, unsigned int); + int (*check_acl)(struct inode *, int, unsigned int); int (*setattr) (struct dentry *, struct iattr *); int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); @@ -414,6 +415,13 @@ otherwise noted. permission: called by the VFS to check for access rights on a POSIX-like filesystem. + May be called in rcu-walk mode (flags & IPERM_RCU). If in rcu-walk + mode, the filesystem must check the permission without blocking or + storing to the inode. + + If a situation is encountered that rcu-walk cannot handle, return + -ECHILD and it will be called again in ref-walk mode. + setattr: called by the VFS to set attributes for a file. This method is called by chmod(2) and related system calls. @@ -534,6 +542,7 @@ struct address_space_operations { sector_t (*bmap)(struct address_space *, sector_t); int (*invalidatepage) (struct page *, unsigned long); int (*releasepage) (struct page *, int); + void (*freepage)(struct page *); ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, loff_t offset, unsigned long nr_segs); struct page* (*get_xip_page)(struct address_space *, sector_t, @@ -660,11 +669,10 @@ struct address_space_operations { releasepage: releasepage is called on PagePrivate pages to indicate that the page should be freed if possible. ->releasepage should remove any private data from the page and clear the - PagePrivate flag. It may also remove the page from the - address_space. If this fails for some reason, it may indicate - failure with a 0 return value. - This is used in two distinct though related cases. The first - is when the VM finds a clean page with no active users and + PagePrivate flag. If releasepage() fails for some reason, it must + indicate failure with a 0 return value. + releasepage() is used in two distinct though related cases. The + first is when the VM finds a clean page with no active users and wants to make it a free page. If ->releasepage succeeds, the page will be removed from the address_space and become free. @@ -679,6 +687,12 @@ struct address_space_operations { need to ensure this. Possibly it can clear the PageUptodate bit if it cannot free private data yet. + freepage: freepage is called once the page is no longer visible in + the page cache in order to allow the cleanup of any private + data. Since it may be called by the memory reclaimer, it + should not assume that the original address_space mapping still + exists, and it should not block. + direct_IO: called by the generic read/write routines to perform direct_IO - that is IO requests which bypass the page cache and transfer data directly between the storage and the @@ -841,9 +855,12 @@ defined: struct dentry_operations { int (*d_revalidate)(struct dentry *, struct nameidata *); - int (*d_hash) (struct dentry *, struct qstr *); - int (*d_compare) (struct dentry *, struct qstr *, struct qstr *); - int (*d_delete)(struct dentry *); + int (*d_hash)(const struct dentry *, const struct inode *, + struct qstr *); + int (*d_compare)(const struct dentry *, const struct inode *, + const struct dentry *, const struct inode *, + unsigned int, const char *, const struct qstr *); + int (*d_delete)(const struct dentry *); void (*d_release)(struct dentry *); void (*d_iput)(struct dentry *, struct inode *); char *(*d_dname)(struct dentry *, char *, int); @@ -854,13 +871,45 @@ struct dentry_operations { dcache. Most filesystems leave this as NULL, because all their dentries in the dcache are valid - d_hash: called when the VFS adds a dentry to the hash table + d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU). + If in rcu-walk mode, the filesystem must revalidate the dentry without + blocking or storing to the dentry, d_parent and d_inode should not be + used without care (because they can go NULL), instead nd->inode should + be used. + + If a situation is encountered that rcu-walk cannot handle, return + -ECHILD and it will be called again in ref-walk mode. + + d_hash: called when the VFS adds a dentry to the hash table. The first + dentry passed to d_hash is the parent directory that the name is + to be hashed into. The inode is the dentry's inode. + + Same locking and synchronisation rules as d_compare regarding + what is safe to dereference etc. + + d_compare: called to compare a dentry name with a given name. The first + dentry is the parent of the dentry to be compared, the second is + the parent's inode, then the dentry and inode (may be NULL) of the + child dentry. len and name string are properties of the dentry to be + compared. qstr is the name to compare it with. + + Must be constant and idempotent, and should not take locks if + possible, and should not or store into the dentry or inodes. + Should not dereference pointers outside the dentry or inodes without + lots of care (eg. d_parent, d_inode, d_name should not be used). + + However, our vfsmount is pinned, and RCU held, so the dentries and + inodes won't disappear, neither will our sb or filesystem module. + ->i_sb and ->d_sb may be used. - d_compare: called when a dentry should be compared with another + It is a tricky calling convention because it needs to be called under + "rcu-walk", ie. without any locks or references on things. - d_delete: called when the last reference to a dentry is - deleted. This means no-one is using the dentry, however it is - still valid and in the dcache + d_delete: called when the last reference to a dentry is dropped and the + dcache is deciding whether or not to cache it. Return 1 to delete + immediately, or 0 to cache the dentry. Default is NULL which means to + always cache a reachable dentry. d_delete must be constant and + idempotent. d_release: called when a dentry is really deallocated @@ -904,14 +953,11 @@ manipulate dentries: the usage count) dput: close a handle for a dentry (decrements the usage count). If - the usage count drops to 0, the "d_delete" method is called - and the dentry is placed on the unused list if the dentry is - still in its parents hash list. Putting the dentry on the - unused list just means that if the system needs some RAM, it - goes through the unused list of dentries and deallocates them. - If the dentry has already been unhashed and the usage count - drops to 0, in this case the dentry is deallocated after the - "d_delete" method is called + the usage count drops to 0, and the dentry is still in its + parent's hash, the "d_delete" method is called to check whether + it should be cached. If it should not be cached, or if the dentry + is not hashed, it is deleted. Otherwise cached dentries are put + into an LRU list to be reclaimed on memory shortage. d_drop: this unhashes a dentry from its parents hash list. A subsequent call to dput() will deallocate the dentry if its |