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

[Minchan Kim <minchan.kim@gmail.com>]

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