Re: [PATCH v3] mm: Fix for negative counter: nr_file_hugepages

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On 08.11.23 18:15, Stefan Roesch wrote:
While qualifiying the 6.4 release, the following warning was detected in
messages:

vmstat_refresh: nr_file_hugepages -15664

The warning is caused by the incorrect updating of the NR_FILE_THPS
counter in the function split_huge_page_to_list. The if case is checking
for folio_test_swapbacked, but the else case is missing the check for
folio_test_pmd_mappable. The other functions that manipulate the counter
like __filemap_add_folio and filemap_unaccount_folio have the
corresponding check.

I have a test case, which reproduces the problem. It can be found here:
   https://github.com/sroeschus/testcase/blob/main/vmstat_refresh/madv.c

The test case reproduces on an XFS filesystem. Running the same test
case on a BTRFS filesystem does not reproduce the problem.

AFAIK version 6.1 until 6.6 are affected by this problem.

Signed-off-by: Stefan Roesch <shr@xxxxxxxxxxxx>
Co-debugged-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
  mm/huge_memory.c | 16 +++++++++-------
  1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 064fbd90822b4..874000f97bfc1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2737,13 +2737,15 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
  			int nr = folio_nr_pages(folio);
xas_split(&xas, folio, folio_order(folio));
-			if (folio_test_swapbacked(folio)) {
-				__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
-							-nr);
-			} else {
-				__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
-							-nr);
-				filemap_nr_thps_dec(mapping);
+			if (folio_test_pmd_mappable(folio)) {
+				if (folio_test_swapbacked(folio)) {
+					__lruvec_stat_mod_folio(folio,
+							NR_SHMEM_THPS, -nr);
+				} else {
+					__lruvec_stat_mod_folio(folio,
+							NR_FILE_THPS, -nr);
+					filemap_nr_thps_dec(mapping);
+				}

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

Yes, that's the current state: update these counters only for (traditional, IOW PMD-sized) THP. What we'll do with non-pmd-sized THP remains to be discussed (Ryan had some ideas).

--
Cheers,

David / dhildenb




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