mm: track vma changes with VM_SOFTDIRTY bit

Pavel reported that in case if vma area get unmapped and then mapped (or
expanded) in-place, the soft dirty tracker won't be able to recognize this
situation since it works on pte level and ptes are get zapped on unmap,
loosing soft dirty bit of course.

So to resolve this situation we need to track actions on vma level, there
VM_SOFTDIRTY flag comes in.  When new vma area created (or old expanded)
we set this bit, and keep it here until application calls for clearing
soft dirty bit.

Thus when user space application track memory changes now it can detect if
vma area is renewed.

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 1 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 13926a5a6901..51958d192a48 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1609,6 +1609,15 @@ out:
 	if (file)
 		uprobe_mmap(vma);
 
+	/*
+	 * New (or expanded) vma always get soft dirty status.
+	 * Otherwise user-space soft-dirty page tracker won't
+	 * be able to distinguish situation when vma area unmapped,
+	 * then new mapped in-place (which must be aimed as
+	 * a completely new data area).
+	 */
+	vma->vm_flags |= VM_SOFTDIRTY;
+
 	return addr;
 
 unmap_and_free_vma:
@@ -2652,6 +2661,7 @@ out:
 	mm->total_vm += len >> PAGE_SHIFT;
 	if (flags & VM_LOCKED)
 		mm->locked_vm += (len >> PAGE_SHIFT);
+	vma->vm_flags |= VM_SOFTDIRTY;
 	return addr;
 }
 
@@ -2916,7 +2926,7 @@ int install_special_mapping(struct mm_struct *mm,
 	vma->vm_start = addr;
 	vma->vm_end = addr + len;
 
-	vma->vm_flags = vm_flags | mm->def_flags | VM_DONTEXPAND;
+	vma->vm_flags = vm_flags | mm->def_flags | VM_DONTEXPAND | VM_SOFTDIRTY;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 
 	vma->vm_ops = &special_mapping_vmops;