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    SubjectRe: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information
    On Tue, Apr 27, 2010 at 08:05:26AM +0900, Minchan Kim wrote:
    > On Tue, Apr 27, 2010 at 7:37 AM, 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>
    > > ---
    > >  mm/ksm.c  |   13 +++++++++++++
    > >  mm/mmap.c |    6 ++++++
    > >  mm/rmap.c |   22 +++++++++++++++++++---
    > >  3 files changed, 38 insertions(+), 3 deletions(-)
    > >
    > > diff --git a/mm/ksm.c b/mm/ksm.c
    > > index 3666d43..baa5b4d 100644
    > > --- a/mm/ksm.c
    > > +++ b/mm/ksm.c
    > > @@ -1674,9 +1674,22 @@ again:
    > >                spin_lock(&anon_vma->lock);
    > >                list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
    > >                        vma = vmac->vma;
    > > +
    > > +                       /* See comment in mm/rmap.c#rmap_walk_anon on locking */
    > > +                       if (anon_vma != vma->anon_vma) {
    > > +                               if (!spin_trylock(&vma->anon_vma->lock)) {
    > > +                                       spin_unlock(&anon_vma->lock);
    > > +                                       goto again;
    > > +                               }
    > > +                       }
    > > +
    > >                        if (rmap_item->address < vma->vm_start ||
    > >                            rmap_item->address >= vma->vm_end)
    > >                                continue;
    > > +
    > > +                       if (anon_vma != vma->anon_vma)
    > > +                               spin_unlock(&vma->anon_vma->lock);
    > > +
    > >                        /*
    > >                         * Initially we examine only the vma which covers this
    > >                         * rmap_item; but later, if there is still work to do,
    > > diff --git a/mm/mmap.c b/mm/mmap.c
    > > index f90ea92..61d6f1d 100644
    > > --- a/mm/mmap.c
    > > +++ b/mm/mmap.c
    > > @@ -578,6 +578,9 @@ again:                      remove_next = 1 + (end > next->vm_end);
    > >                }
    > >        }
    > >
    > > +       if (vma->anon_vma)
    > > +               spin_lock(&vma->anon_vma->lock);
    > > +
    > >        if (root) {
    > >                flush_dcache_mmap_lock(mapping);
    > >                vma_prio_tree_remove(vma, root);
    > > @@ -620,6 +623,9 @@ again:                      remove_next = 1 + (end > next->vm_end);
    > >        if (mapping)
    > >                spin_unlock(&mapping->i_mmap_lock);
    > >
    > > +       if (vma->anon_vma)
    > > +               spin_unlock(&vma->anon_vma->lock);
    > > +
    > >        if (remove_next) {
    > >                if (file) {
    > >                        fput(file);
    > > diff --git a/mm/rmap.c b/mm/rmap.c
    > > index 85f203e..bc313a6 100644
    > > --- a/mm/rmap.c
    > > +++ b/mm/rmap.c
    > > @@ -1368,15 +1368,31 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
    > >         * are holding mmap_sem. Users without mmap_sem are required to
    > >         * take a reference count to prevent the anon_vma disappearing
    > >         */
    > > +retry:
    > >        anon_vma = page_anon_vma(page);
    > >        if (!anon_vma)
    > >                return ret;
    > >        spin_lock(&anon_vma->lock);
    > >        list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
    > >                struct vm_area_struct *vma = avc->vma;
    > > -               unsigned long address = vma_address(page, vma);
    > > -               if (address == -EFAULT)
    > > -                       continue;
    > > +               unsigned long address;
    > > +
    > > +               /*
    > > +                * Guard against deadlocks by not spinning against
    > > +                * vma->anon_vma->lock. If contention is found, release our
    > > +                * lock and try again until VMA list can be traversed without
    > > +                * contention.
    > > +                */
    > > +               if (anon_vma != vma->anon_vma) {
    > > +                       if (!spin_trylock(&vma->anon_vma->lock)) {
    > > +                               spin_unlock(&anon_vma->lock);
    > > +                               goto retry;
    > > +                       }
    > > +               }
    > > +               address = vma_address(page, vma);
    > > +               if (anon_vma != vma->anon_vma)
    > > +                       spin_unlock(&vma->anon_vma->lock);
    > > +
    >
    > if (address == -EFAULT)
    > continue;
    >

    Correct. Thanks.

    > >                ret = rmap_one(page, vma, address, arg);
    > >                if (ret != SWAP_AGAIN)
    > >                        break;
    > > --
    > > 1.6.5
    > >
    > >
    >

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