mm-khugepaged-fix-nr_file_pages-and-nr_shmem-in-collapse_file.patch removed from -mm tree

The quilt patch titled
Subject: mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in collapse_file()
has been removed from the -mm tree. Its filename was
mm-khugepaged-fix-nr_file_pages-and-nr_shmem-in-collapse_file.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Shakeel Butt <[email protected]>
Subject: mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in collapse_file()
Date: Thu, 29 Jan 2026 20:29:25 -0800

In META's fleet, we observed high-level cgroups showing zero file memcg stats while their descendants had non-zero values. Investigation using drgn revealed that these parent cgroups actually had negative file stats, aggregated from their children.

This issue became more frequent after deploying thp-always more widely, pointing to a correlation with THP file collapsing. The root cause is that collapse_file() assumes old folios and the new THP belong to the same node and memcg. When this assumption breaks, stats become skewed. The bug affects not just memcg stats but also per-numa stats, and not just NR_FILE_PAGES but also NR_SHMEM.

The assumption breaks in scenarios such as:

1. Small folios allocated on one node while the THP gets allocated on a different node.

2. A package downloader running in one cgroup populates the page cache, while a job in a different cgroup executes the downloaded binary.

3. A file shared between processes in different cgroups, where one process faults in the pages and khugepaged (or madvise(COLLAPSE)) collapses them on behalf of the other.

Fix the accounting by explicitly incrementing stats for the new THP and decrementing stats for the old folios being replaced.

---

 mm/khugepaged.c |   16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

@@ -2195,16 +2195,13 @@ immap_locked:
 		xas_lock_irq(&xas);
 	}
 
-	if (is_shmem)
+	if (is_shmem) {
+		lruvec_stat_mod_folio(new_folio, NR_SHMEM, HPAGE_PMD_NR);
 		lruvec_stat_mod_folio(new_folio, NR_SHMEM_THPS, HPAGE_PMD_NR);
-	else
+	} else {
 		lruvec_stat_mod_folio(new_folio, NR_FILE_THPS, HPAGE_PMD_NR);
-
-	if (nr_none) {
-		lruvec_stat_mod_folio(new_folio, NR_FILE_PAGES, nr_none);
-		/* nr_none is always 0 for non-shmem. */
-		lruvec_stat_mod_folio(new_folio, NR_SHMEM, nr_none);
 	}
+	lruvec_stat_mod_folio(new_folio, NR_FILE_PAGES, HPAGE_PMD_NR);
 
 	/*
 	 * Mark new_folio as uptodate before inserting it into the

@@ -2238,6 +2235,11 @@ immap_locked:
 	 */
 	list_for_each_entry_safe(folio, tmp, &pagelist, lru) {
 		list_del(&folio->lru);
+		lruvec_stat_mod_folio(folio, NR_FILE_PAGES,
+				      -folio_nr_pages(folio));
+		if (is_shmem)
+			lruvec_stat_mod_folio(folio, NR_SHMEM,
+					      -folio_nr_pages(folio));
 		folio->mapping = NULL;
 		folio_clear_active(folio);
 		folio_clear_unevictable(folio);

_

Patches currently in -mm which might be from [email protected] are