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    SubjectRe: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information
    From
    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;

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



    --
    Kind regards,
    Minchan Kim
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