+ mm-mglru-fix-and-remove-redundant-unevictable-folio-handling.patch added to mm-new branch

The patch titled
     Subject: mm/mglru: fix and remove redundant unevictable folio handling
has been added to the -mm mm-new branch.  Its filename is
     mm-mglru-fix-and-remove-redundant-unevictable-folio-handling.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mglru-fix-and-remove-redundant-unevictable-folio-handling.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Kairui Song <[email protected]>
Subject: mm/mglru: fix and remove redundant unevictable folio handling
Date: Wed, 12 Aug 2026 20:22:39 +0800

sort_folio() has a shortcut for moving folios that are no longer evictable
but are still sitting on a generation list.  However, this shortcut is
buggy.  It does not follow the PG_lru usage convention, and it has a more
serious issue.

Unevictable folios are not threaded on lists[LRU_UNEVICTABLE], so that
folio->lru can be reused to hold folio->mlock_count (see the comment in
lruvec_init()).  Hence lruvec_add_folio() skips the list_add() for them,
and every other place that turns a folio unevictable initialises
mlock_count explicitly: lru_add() sets it to 0, __mlock_folio() and
__mlock_new_folio() set it to !!folio_test_mlocked(folio).  sort_folio()
sets nothing, and the lru_gen_del_folio() right above it may have already
poisoned folio->lru via list_del(), so mlock_count ends up aliasing
LIST_POISON2, which reads as 0x122, i.e.  290.  The result is user
visible.  On munlock, __munlock_folio() decrements that bogus count, finds
it still non-zero and bails out before clearing PG_mlocked, so the folio
remains unevictable and the Mlocked accounting stays inflated until the
folio is freed.

The shortcut also touches the LRU flags in the wrong order.  It calls
lru_gen_del_folio() while PG_lru is still set, so a concurrent
folio_test_clear_lru() (e.g.  compaction, folio_isolate_lru()) can succeed
on a folio that has already been taken off the generation list, which may
lead to unexpected behavior.

So fix it by isolating them as common folios and letting the generic
shrink path cull them.  This matches the classical LRU behavior, and there
should be no visible effect on the generic eviction or isolation behavior.

There is no performance concern either, such a folio goes through this
once, and then it is off the generation lists for good.

Link: https://lore.kernel.org/[email protected]
Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation")
Signed-off-by: Kairui Song <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jan Alexander Steffens (heftig) <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oleksandr Natalenko <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Steven Barrett <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Cc: Wei Xu <[email protected]>
Cc: Yuanchu Xie <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/vmscan.c |   19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

--- a/mm/vmscan.c~mm-mglru-fix-and-remove-redundant-unevictable-folio-handling
+++ a/mm/vmscan.c
@@ -4649,7 +4649,6 @@ void lru_gen_reparent_memcg(struct mem_c
 static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_control *sc,
 		       int tier_idx)
 {
-	bool success;
 	int gen = folio_lru_gen(folio);
 	int type = folio_is_file_lru(folio);
 	int zone = folio_zonenum(folio);
@@ -4661,15 +4660,9 @@ static bool sort_folio(struct lruvec *lr
 
 	VM_WARN_ON_ONCE_FOLIO(gen >= MAX_NR_GENS, folio);
 
-	/* unevictable */
-	if (!folio_evictable(folio)) {
-		success = lru_gen_del_folio(lruvec, folio, true);
-		VM_WARN_ON_ONCE_FOLIO(!success, folio);
-		folio_set_unevictable(folio);
-		lruvec_add_folio(lruvec, folio);
-		__count_vm_events(UNEVICTABLE_PGCULLED, delta);
-		return true;
-	}
+	/* unevictable: let it through and the generic path will cull it */
+	if (!folio_evictable(folio))
+		return false;
 
 	/* promoted */
 	if (gen != lru_gen_from_seq(lrugen->min_seq[type])) {
@@ -4922,11 +4915,9 @@ retry:
 	list_for_each_entry_safe_reverse(folio, next, &list, lru) {
 		DEFINE_MIN_SEQ(lruvec);
 
-		if (!folio_evictable(folio)) {
-			list_del(&folio->lru);
-			folio_putback_lru(folio);
+		/* move_folios_to_lru() culls unevictable folios via folio_putback_lru() */
+		if (!folio_evictable(folio))
 			continue;
-		}
 
 		/* retry folios that may have missed folio_rotate_reclaimable() */
 		if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
_

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

mm-mglru-fix-and-remove-redundant-unevictable-folio-handling.patch