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
>
