FAILED: patch "[PATCH] mm/mglru: fix and remove redundant unevictable folio handling" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x f7e698e326b239a91ea15844817551921209e826
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090304-economist-cost-ccd2@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f7e698e326b239a91ea15844817551921209e826 Mon Sep 17 00:00:00 2001
From: Kairui Song <[email protected]>
Date: Wed, 12 Aug 2026 20:22:39 +0800
Subject: [PATCH] mm/mglru: fix and remove redundant unevictable folio handling

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.

index 6c35f7e21465..c1404a59523d 100644
@@ -4649,7 +4649,6 @@ void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent,
 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 *lruvec, struct folio *folio, struct scan_c
 
 	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 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	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) &&