[PATCH 05/25] mm/fbatch: lru_add_del_folio()+folio_add_lru() after clear_lru()

[Hugh Dickins]

Most callers of folio_test_clear_lru() then proceed to remove the folio
from its lru, and add it back at the end when they're done (if still in
use). But isolate_migratepages_block() and check_move_unevictable_pages()
sometimes decide against, and release immediately with a folio_set_lru().

Which usually works fine: but there's now a small chance that while they
held the folio with lru bit cleared, an lru_add fbatch drain came along,
and had to skip that folio because its lru bit was transiently cleared
(previously, the lru_add fbatch drain relied on finding lru bit never yet
set). This risks leaving that folio off lru, unreclaimable until freed.

Fix such cases by trying lru_add_del_folio() (which only takes action and
returns true if the folio was on an lru_add fbatch), then folio_add_lru()
if it succeeded: invalidating the old fbatch slot, appending in a new one.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/compaction.c | 13 +++++++++++--
 mm/vmscan.c     | 16 +++++++++-------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index a049415512c6..9e045a90ba21 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1204,7 +1204,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				     !cc->alloc_contig)) {
 				low_pfn += folio_nr_pages(folio) - 1;
 				nr_scanned += folio_nr_pages(folio) - 1;
-				folio_set_lru(folio);
+				if (lru_add_del_folio(folio)) {
+					lruvec_unlock_irqrestore(locked, flags);
+					folio_add_lru(folio);
+					locked = NULL;
+				} else {
+					folio_set_lru(folio);
+				}
 				goto isolate_fail_put;
 			}
 		}
@@ -1293,7 +1299,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 	if (locked)
 		lruvec_unlock_irqrestore(locked, flags);
 	if (folio) {
-		folio_set_lru(folio);
+		if (lru_add_del_folio(folio))
+			folio_add_lru(folio);
+		else
+			folio_set_lru(folio);
 		folio_put(folio);
 	}
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4ca9775ceee8..7da12ffbd3bd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -8021,17 +8021,19 @@ void check_move_unevictable_folios(struct folio_batch *fbatch)
 			folio_clear_unevictable(folio);
 			lruvec_add_folio(lruvec, folio);
 			pgrescued += nr_pages;
+		} else if (lru_add_del_folio(folio)) {
+			lruvec_unlock_irq(lruvec);
+			folio_add_lru(folio);
+			lruvec = NULL;
 		}
-		folio_set_lru(folio);
+		if (lruvec)
+			folio_set_lru(folio);
 	}
 
-	if (lruvec) {
-		__count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
-		__count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
+	if (lruvec)
 		lruvec_unlock_irq(lruvec);
-	} else if (pgscanned) {
-		count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
-	}
+	count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
+	count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
 }
 EXPORT_SYMBOL_GPL(check_move_unevictable_folios);
 
-- 
2.51.0