The patch titled Subject: shmem: fix a race between shmem_unused_huge_shrink and shmem_evict_inode has been added to the -mm tree. Its filename is shmem-fix-a-race-between-shmem_unused_huge_shrink-and-shmem_evict_inode.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/shmem-fix-a-race-between-shmem_unu... and later at https://ozlabs.org/~akpm/mmotm/broken-out/shmem-fix-a-race-between-shmem_unu...

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next and is updated there every 3-4 working days

------------------------------------------------------ From: Gang Li ligang.bdlg@bytedance.com Subject: shmem: fix a race between shmem_unused_huge_shrink and shmem_evict_inode

Fix a data race in commit 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure").

Here are call traces causing race:

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

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 ^ ^ | | evict pos (drop)

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

Fixed by moving inodes back to global list before we put them.

Link: https://lkml.kernel.org/r/20211125064502.99983-1-ligang.bdlg@bytedance.com Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure") Signed-off-by: Gang Li ligang.bdlg@bytedance.com Reviewed-by: Muchun Song songmuchun@bytedance.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/shmem.c | 36 ++++++++++++++++++++---------------- 1 file changed, 20 insertions(+), 16 deletions(-)

--- a/mm/shmem.c~shmem-fix-a-race-between-shmem_unused_huge_shrink-and-shmem_evict_inode +++ a/mm/shmem.c @@ -554,7 +554,7 @@ static unsigned long shmem_unused_huge_s struct shmem_inode_info *info; struct page *page; unsigned long batch = sc ? sc->nr_to_scan : 128; - int removed = 0, split = 0; + int split = 0;

if (list_empty(&sbinfo->shrinklist)) return SHRINK_STOP; @@ -569,7 +569,6 @@ static unsigned long shmem_unused_huge_s /* inode is about to be evicted */ if (!inode) { list_del_init(&info->shrinklist); - removed++; goto next; }

@@ -577,12 +576,12 @@ static unsigned long shmem_unused_huge_s 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: + sbinfo->shrinklist_len--; if (!--batch) break; } @@ -602,7 +601,7 @@ next: 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 +615,43 @@ next: }

/* - * 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: + /* + * Make sure the inode is either on the global list or deleted from + * any local list before iput() since it could be deleted in another + * thread once we put the inode (then the local list is corrupted). + */ + spin_lock(&sbinfo->shrinklist_lock); + list_move(&info->shrinklist, &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; }

_

Patches currently in -mm which might be from ligang.bdlg@bytedance.com are

shmem-fix-a-race-between-shmem_unused_huge_shrink-and-shmem_evict_inode.patch vmscan-make-drop_slab_node-static.patch