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    SubjectRe: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information
    On Tue, 27 Apr 2010 12:50:40 +0900
    KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

    > On Tue, 27 Apr 2010 09:07:06 +0900
    > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    >
    > > On Mon, 26 Apr 2010 23:37:58 +0100
    > > Mel Gorman <mel@csn.ul.ie> wrote:
    > >
    > > > vma_adjust() is updating anon VMA information without any locks taken.
    > > > In contrast, file-backed mappings use the i_mmap_lock and 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 can be more than one anon_vma->lock
    > > > that can be taken in a anon_vma_chain but a second lock cannot be spinned
    > > > upon in case of deadlock. Instead, the rmap walker tries to take locks of
    > > > different anon_vma's. If the attempt fails, the operation is restarted.
    > > >
    > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    > >
    > > Ok, acquiring vma->anon_vma->spin_lock always sounds very safe.
    > > (but slow.)
    > >
    > > I'll test this, too.
    > >
    > > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > >
    >
    > Sorry. reproduced. It seems the same bug before patch.
    > mapcount 1 -> unmap -> remap -> mapcount 0. And it was SwapCache.
    >
    Sorry again. It was _not_ SwapCache. Just SwapBacked.

    Thanks,
    -Kame



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