Re: [PATCH v2 0/2] mm: fix PMD level mTHP accounting bugs

[Andrew Morton]

On Fri, 17 Jul 2026 00:44:58 -0600 Nico Pache <npache@xxxxxxxxxx> wrote:

> While running selftests I noticed the PMD level per-mTHP stats (nr_anon)
> remained elevated after each run. After further investigation I noticed
> this accounting error occurs for both the migration.private_anon_htlb_test
> and the HMM tests.
> 
> In the HMM case this is due to folio_add_new_anon_rmap() incrementing the
> mTHP stats, but never containing a corresponding decrement in
> free_zone_device_folio(). We solve this by making sure to decrement the
> counter when freeing device memory.
> 
> In the migration case, we are incrementing this counter without first
> checking whether this folio is a hugetlb folio, which relies on a separate
> accounting system. We solve this by adding the proper hugetlb check before
> incrementing this counter.
> 
> With these changes in place, the two tests no longer cause elevated PMD
> level accounting issues.

Thanks, I've updated mm.git's mm-hotfixes-unstable branch to this
version.

Sashiko is worried about the existing code ("list corruption or a
use-after-free panic"):
	https://sashiko.dev/#/patchset/20260717064502.1980173-1-npache@xxxxxxxxxx

> 
> V2 Changes:
>  - add RB from Zi
>  - Drop unnecessary check in __folio_migrate_folio(). HugeTLB has its own
>    migration system in hugetlbfs_migrate_folio().

Here's how v2 altered mm.git:

 mm/migrate.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/mm/migrate.c~b
+++ a/mm/migrate.c
@@ -624,8 +624,7 @@ static int __folio_migrate_mapping(struc
 	 */
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
-	if (folio_test_anon(folio) && folio_test_large(folio) &&
-	    !folio_test_hugetlb(folio))
+	if (folio_test_anon(folio) && folio_test_large(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))
_