ext4fs: Add ext4 extent cache for read operations

In an ext4 filesystem, the inode corresponding to a file has a 60-byte
area which contains an extent header structure and up to 4 extent
structures (5 x 12 bytes).

For files that need more than 4 extents to be represented (either files
larger than 4 x 128MB = 512MB or smaller files but very fragmented),
ext4 creates extent index structures. Each extent index points to a 4KB
physical block where one extent header and additional 340 extents could
be stored.

The current u-boot ext4 code is very inefficient when it tries to load a
file which has extent indexes. For each logical file block the code will
read over and over again the same blocks of 4096 bytes from the disk.

Since the extent tree in a file is always the same, we can cache the
extent structures in memory before actually starting to read the file.

This patch creates a simple linked list of structures holding information
about all the extents used to represent a file. The list is sorted by
the logical block number (ee_block) so that we can easily find the
proper extent information for any file block.

Without this patch, a 69MB file which had just one extent index pointing
to a block with another 6 extents was read in approximately 3 minutes.
With this patch applied the same file can be read in almost 20 seconds.

Signed-off-by: Ionut Nicu <ioan.nicu.ext@nsn.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/ext4/ext4_common.h b/fs/ext4/ext4_common.h
index 5fa1719f2e..a9fd8c6573 100644
--- a/fs/ext4/ext4_common.h
+++ b/fs/ext4/ext4_common.h
@@ -57,6 +57,9 @@ int ext4fs_find_file(const char *path, struct ext2fs_node *rootnode,
 int ext4fs_iterate_dir(struct ext2fs_node *dir, char *name,
 			struct ext2fs_node **fnode, int *ftype);
 
+int ext4fs_build_extent_cache(struct ext2_inode *inode);
+void ext4fs_free_extent_cache(void);
+
 #if defined(CONFIG_EXT4_WRITE)
 uint32_t ext4fs_div_roundup(uint32_t size, uint32_t n);
 int ext4fs_checksum_update(unsigned int i);