[PATCH] mm/swap_state: update zswap LRU's protection range with the folio locked

[Nhat Pham <nphamcs@gmail.com>]

Move the zswap LRU protection range update above the swap_read_folio()
call, and only when a new page is allocated. This is the case where
(z)swapin could happen, which is a signal that the zswap shrinker should
be more conservative with its reclaiming action.

It also prevents a race, in which folio migration can clear the
memcg_data of the now unlocked folio, resulting in a warning in the
inlined folio_lruvec() call.

Reported-by: syzbot+17a611d10af7d18a7092@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000ae47f90610803260@google.com/
Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/swap_state.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index e671266ad772..7255c01a1e4e 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -680,9 +680,10 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
 	/* The page was likely read above, so no need for plugging here */
 	folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
 					&page_allocated, false);
-	if (unlikely(page_allocated))
+	if (unlikely(page_allocated)) {
+		zswap_folio_swapin(folio);
 		swap_read_folio(folio, false, NULL);
-	zswap_folio_swapin(folio);
+	}
 	return folio;
 }
 
@@ -855,9 +856,10 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
 	/* The folio was likely read above, so no need for plugging here */
 	folio = __read_swap_cache_async(targ_entry, gfp_mask, mpol, targ_ilx,
 					&page_allocated, false);
-	if (unlikely(page_allocated))
+	if (unlikely(page_allocated)) {
+		zswap_folio_swapin(folio);
 		swap_read_folio(folio, false, NULL);
-	zswap_folio_swapin(folio);
+	}
 	return folio;
 }
 

base-commit: 91f3daa1765ee4e0c89987dc25f72c40f07af34d
-- 
2.39.3