Re: [PATCH v2] shmem: move spinlock to the front of iput

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On Wed, Nov 24, 2021 at 11:08 AM Gang Li <ligang.bdlg@xxxxxxxxxxxxx> wrote:
>
> This patch fixes a data race in commit 779750d20b93 ("shmem: split huge pages
> beyond i_size under memory pressure").
>
> Call Trace 1:
>  shmem_unused_huge_shrink+0x3ae/0x410
>  ? __list_lru_walk_one.isra.5+0x33/0x160
>  super_cache_scan+0x17c/0x190
>  shrink_slab.part.55+0x1ef/0x3f0
>  shrink_node+0x10e/0x330
>  kswapd+0x380/0x740
>  kthread+0xfc/0x130
>  ? mem_cgroup_shrink_node+0x170/0x170
>  ? kthread_create_on_node+0x70/0x70
>  ret_from_fork+0x1f/0x30
>
> Call Trace 2:
>  shmem_evict_inode+0xd8/0x190
>  evict+0xbe/0x1c0
>  do_unlinkat+0x137/0x330
>  do_syscall_64+0x76/0x120
>  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>
> iput out of sbinfo->shrinklist_lock will let shmem_evict_inode grab
> and delete the inode, which will berak the consistency between
> shrinklist_len and shrinklist. The simultaneous deletion of adjacent
> elements in the local list "list" by shmem_unused_huge_shrink and
> shmem_evict_inode will also break the list.
>
> Fix it by moving shrinklist_lock to the front of iput.
>
> Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
> Signed-off-by: Gang Li <ligang.bdlg@xxxxxxxxxxxxx>

I suggest changing the subject to "shmem: fix a race between
shmem_unused_huge_shrink and shmem_evict_inode". And please Cc
stable@xxxxxxxxxxxxxxx as well.

I do not know if others see what happens here, here is a simple explanation.

Image there are 3 items in the local list (@list).

In the first traversal, A is not deleted from @list.

  1) A->B->C
     ^
     |
    pos (leave)

In the second traversal, B is deleted from @list. Concurrently, A is
deleted from
@list through shmem_evict_inode() since last reference counter of
inode is dropped
by other thread. Then the @list is corrupted.

  2) A->B->C
        ^
        |
       pos (drop)

We should make sure the item is either on the global list or deleted from
any local list before iput().

> ---
>
> Changes in v2:
> - Move spinlock to the front of iput instead of changing lock type
>   since iput will call evict which may cause deadlock by requesting
>   shrinklist_lock.
> - Add call trace in commit message.
>
> v1: https://lore.kernel.org/lkml/20211122064126.76734-1-ligang.bdlg@xxxxxxxxxxxxx/
>
> ---
>  mm/shmem.c | 30 +++++++++++++++---------------
>  1 file changed, 15 insertions(+), 15 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 9023103ee7d8..2f70a16fc588 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -569,7 +569,6 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
>                 /* inode is about to be evicted */
>                 if (!inode) {
>                         list_del_init(&info->shrinklist);
> -                       removed++;
>                         goto next;
>                 }
>
> @@ -577,15 +576,16 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
>                 if (round_up(inode->i_size, PAGE_SIZE) ==
>                                 round_up(inode->i_size, HPAGE_PMD_SIZE)) {
>                         list_move(&info->shrinklist, &to_remove);
> -                       removed++;
>                         goto next;
>                 }
>
>                 list_move(&info->shrinklist, &list);
>  next:
> +               removed++;
>                 if (!--batch)
>                         break;
>         }
> +       sbinfo->shrinklist_len -= removed;
>         spin_unlock(&sbinfo->shrinklist_lock);
>
>         list_for_each_safe(pos, next, &to_remove) {
> @@ -602,7 +602,7 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
>                 inode = &info->vfs_inode;
>
>                 if (nr_to_split && split >= nr_to_split)
> -                       goto leave;
> +                       goto move_back;
>
>                 page = find_get_page(inode->i_mapping,
>                                 (inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT);
> @@ -616,38 +616,38 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
>                 }
>
>                 /*
> -                * Leave the inode on the list if we failed to lock
> -                * the page at this time.
> +                * Move the inode on the list back to shrinklist if we failed
> +                * to lock the page at this time.
>                  *
>                  * Waiting for the lock may lead to deadlock in the
>                  * reclaim path.
>                  */
>                 if (!trylock_page(page)) {
>                         put_page(page);
> -                       goto leave;
> +                       goto move_back;
>                 }
>
>                 ret = split_huge_page(page);
>                 unlock_page(page);
>                 put_page(page);
>
> -               /* If split failed leave the inode on the list */
> +               /* If split failed move the inode on the list back to shrinklist */
>                 if (ret)
> -                       goto leave;
> +                       goto move_back;
>
>                 split++;
>  drop:
>                 list_del_init(&info->shrinklist);
> -               removed++;
> -leave:
> +               goto put;
> +move_back:

I think we should add a comment here to notice why it's
necessary to add @pos to the global list before iput().

> +               spin_lock(&sbinfo->shrinklist_lock);
> +               list_move(pos, &sbinfo->shrinklist);

I suggest using `list_move(&info->shrinklist, &sbinfo->shrinklist)`
here, it's more consistent with previous list_del_init().

> +               sbinfo->shrinklist_len++;
> +               spin_unlock(&sbinfo->shrinklist_lock);
> +put:
>                 iput(inode);
>         }
>
> -       spin_lock(&sbinfo->shrinklist_lock);
> -       list_splice_tail(&list, &sbinfo->shrinklist);
> -       sbinfo->shrinklist_len -= removed;
> -       spin_unlock(&sbinfo->shrinklist_lock);
> -
>         return split;
>  }
>
> --
> 2.20.1
>




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