From d9104d1ca9662498339c0de975b4666c30485f4e Mon Sep 17 00:00:00 2001 From: Cyrill Gorcunov Date: Wed, 11 Sep 2013 14:22:24 -0700 Subject: 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 Signed-off-by: Cyrill Gorcunov Cc: Andy Lutomirski Cc: Matt Mackall Cc: Xiao Guangrong Cc: Marcelo Tosatti Cc: KOSAKI Motohiro Cc: Stephen Rothwell Cc: Peter Zijlstra Cc: "Aneesh Kumar K.V" Cc: Rob Landley Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/vm/soft-dirty.txt | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'Documentation/vm/soft-dirty.txt') diff --git a/Documentation/vm/soft-dirty.txt b/Documentation/vm/soft-dirty.txt index 9a12a5956bc0..55684d11a1e8 100644 --- a/Documentation/vm/soft-dirty.txt +++ b/Documentation/vm/soft-dirty.txt @@ -28,6 +28,13 @@ This is so, since the pages are still mapped to physical memory, and thus all the kernel does is finds this fact out and puts both writable and soft-dirty bits on the PTE. + While in most cases tracking memory changes by #PF-s is more than enough +there is still a scenario when we can lose soft dirty bits -- a task +unmaps a previously mapped memory region and then maps a new one at exactly +the same place. When unmap is called, the kernel internally clears PTE values +including soft dirty bits. To notify user space application about such +memory region renewal the kernel always marks new memory regions (and +expanded regions) as soft dirty. This feature is actively used by the checkpoint-restore project. You can find more details about it on http://criu.org -- cgit v1.2.3-59-g8ed1b