[PATCH] mm: cachestat: avoid bogus workingset test during swapping & invalidation races

[Johannes Weiner <hannes@cmpxchg.org>]

When cachestat against shmem races with swapping and invalidation, the
shadow entry might not exist: swapout IO is still in progress and
we're before __remove_mapping; or swapin/invalidation/swapoff has
removed the shadow from swapcache after we saw a shmem swap entry.

This will send a NULL to workingset_test_recent(). The latter purely
operates on pointer bits, so it won't crash - node 0, memcg ID 0,
eviction timestamp 0, etc. are all valid inputs - but it's a bogus
test. In theory that could result in a false "recently evicted" count.

Such a false positive wouldn't be the end of the world. But for code
clarity and (future) robustness, be explicit about this case.

Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/filemap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 222adac7c9c5..a07c27df7eab 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4199,6 +4199,9 @@ static void filemap_cachestat(struct address_space *mapping,
 				swp_entry_t swp = radix_to_swp_entry(folio);
 
 				shadow = get_shadow_from_swap_cache(swp);
+				/* can race with swapping & invalidation */
+				if (!shadow)
+					goto resched;
 			}
 #endif
 			if (workingset_test_recent(shadow, true, &workingset))
-- 
2.44.0