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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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