[PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

In __folio_remove_rmap(), a large folio is added to deferred split list
if any page in a folio loses its final mapping. It is possible that
the folio is unmapped fully, but it is unnecessary to add the folio
to deferred split list at all. Fix it by checking folio mapcount before
adding a folio to deferred split list.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/rmap.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40dffad..d599a772e282 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		enum rmap_level level)
 {
 	atomic_t *mapped = &folio->_nr_pages_mapped;
-	int last, nr = 0, nr_pmdmapped = 0;
+	int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
 	enum node_stat_item idx;
 
 	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
@@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 			break;
 		}
 
-		atomic_sub(nr_pages, &folio->_large_mapcount);
+		mapcount = atomic_sub_return(nr_pages,
+					     &folio->_large_mapcount) + 1;
 		do {
 			last = atomic_add_negative(-1, &page->_mapcount);
 			if (last) {
@@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		 * is still mapped.
 		 */
 		if (folio_test_large(folio) && folio_test_anon(folio))
-			if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
+			if ((level == RMAP_LEVEL_PTE &&
+			     mapcount != 0) ||
+			    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))
 				deferred_split_folio(folio);
 	}
 

base-commit: ed7c95c95397baff9b40ba9b0919933eee2d7960
-- 
2.43.0