Re: [PATCH v4] shmem: fix a race between shmem_unused_huge_shrink and shmem_evict_inode

On Thu, Nov 25, 2021 at 11:12 AM Gang Li ligang.bdlg@bytedance.com wrote:

This patch fixes 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.

Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure") Signed-off-by: Gang Li ligang.bdlg@bytedance.com

You have forgotten my Reviewed-by and Kirill A. Shutemov's Acked-by as well as Cc: stable@vger.kernel.org.

---

mm/shmem.c | 34 +++++++++++++++++++--------------- 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c index 9023103ee7d8..e6ccb2a076ff 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++;
  

I believe there is a warning about @removed since it's unused.

                    goto next;
            }

@@ -577,12 +576,12 @@ 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:

•           sbinfo->shrinklist_len--;
            if (!--batch)
                    break;
    }
  

@@ -602,7 +601,7 @@ 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 +615,43 @@ 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:

•           /*
  

•           * 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;
  

}

-- 2.20.1