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Subject[PATCH v2] shmem: move spinlock to the front of iput
Date
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@bytedance.com>
---

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@bytedance.com/

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