[PATCH 2/2] mm/migrate: exclude hugetlb folios from MTHP_STAT_NR_ANON accounting

[Nico Pache]

__folio_migrate_mapping() increments MTHP_STAT_NR_ANON for the
destination folio when `folio_test_anon(folio) && folio_test_large(folio)`
is true. However, hugetlb folios satisfy both conditions despite having a
completely separate accounting system; they use hugetlb_add_anon_rmap()
which does not touch mTHP stats, and their free path also bypasses the
mTHP decrement in __free_pages_prepare().

This causes MTHP_STAT_NR_ANON to be incremented on each hugetlb
migration without a corresponding decrement, permanently inflating the
nr_anon counter.

Add a !folio_test_hugetlb() check to both places in
__folio_migrate_mapping() so that only actual mTHP folios are counted.

Fixes: 5d65c8d758f2 ("mm: count the number of anonymous THPs per size")
Co-developed-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
 mm/migrate.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index d9b23909d716..9fd50ea25d2d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -590,7 +590,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 		/* No turning back from here */
 		newfolio->index = folio->index;
 		newfolio->mapping = folio->mapping;
-		if (folio_test_anon(folio) && folio_test_large(folio))
+		if (folio_test_anon(folio) && folio_test_large(folio) &&
+		    !folio_test_hugetlb(folio))
 			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
 		if (folio_test_swapbacked(folio))
 			__folio_set_swapbacked(newfolio);
@@ -623,7 +624,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 	 */
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
-	if (folio_test_anon(folio) && folio_test_large(folio))
+	if (folio_test_anon(folio) && folio_test_large(folio) &&
+	    !folio_test_hugetlb(folio))
 		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
 	folio_ref_add(newfolio, nr); /* add cache reference */
 	if (folio_test_swapbacked(folio))
-- 
2.54.0