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    SubjectRe: [BUGFIX][mm][PATCH] fix migration race in rmap_walk
    On Sat, Apr 24, 2010 at 11:02:00AM +0900, KAMEZAWA Hiroyuki wrote:
    > On Fri, 23 Apr 2010 16:58:01 +0100
    > Mel Gorman <> wrote:
    > > > I had considered this idea as well as it is vaguely similar to how zones get
    > > > resized with a seqlock. I was hoping that the existing locking on anon_vma
    > > > would be usable by backing off until uncontended but maybe not so lets
    > > > check out this approach.
    > > >
    > >
    > > A possible combination of the two approaches is as follows. It uses the
    > > anon_vma lock mostly except where the anon_vma differs between the page
    > > and the VMAs being walked in which case it uses the seq counter. I've
    > > had it running a few hours now without problems but I'll leave it
    > > running at least 24 hours.
    > >
    > ok, I'll try this, too.
    > > ==== CUT HERE ====
    > > mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information by protecting against vma_adjust with a combination of locks and seq counter
    > >
    > > vma_adjust() is updating anon VMA information without any locks taken.
    > > In constract, file-backed mappings use the i_mmap_lock. This lack of
    > > locking can result in races with page migration. During rmap_walk(),
    > > vma_address() can return -EFAULT for an address that will soon be valid.
    > > This leaves a dangling migration PTE behind which can later cause a
    > > BUG_ON to trigger when the page is faulted in.
    > >
    > > With the recent anon_vma changes, there is no single anon_vma->lock that
    > > can be taken that is safe for rmap_walk() to guard against changes by
    > > vma_adjust(). Instead, a lock can be taken on one VMA while changes
    > > happen to another.
    > >
    > > What this patch does is protect against updates with a combination of
    > > locks and seq counters. First, the vma->anon_vma lock is taken by
    > > vma_adjust() and the sequence counter starts. The lock is released and
    > > the sequence ended when the VMA updates are complete.
    > >
    > > The lock serialses rmap_walk_anon when the page and VMA share the same
    > > anon_vma. Where the anon_vmas do not match, the seq counter is checked.
    > > If a change is noticed, rmap_walk_anon drops its locks and starts again
    > > from scratch as the VMA list may have changed. The dangling migration
    > > PTE bug was not triggered after several hours of stress testing with
    > > this patch applied.
    > >
    > > [ Use of a seq counter]
    > > Signed-off-by: Mel Gorman <>
    > I think this patch is nice!

    It looks nice but it still broke after 28 hours of running. The
    seq-counter is still insufficient to catch all changes that are made to
    the list. I'm beginning to wonder if a) this really can be fully safely
    locked with the anon_vma changes and b) if it has to be a spinlock to
    catch the majority of cases but still a lazy cleanup if there happens to
    be a race. It's unsatisfactory and I'm expecting I'll either have some
    insight to the new anon_vma changes that allow it to be locked or Rik
    knows how to restore the original behaviour which as Andrea pointed out
    was safe.

    > Acked-by: KAMEZAWA Hiroyuki <>

    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab

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