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



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



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