+ mm-mglru-lift-the-forced-protection-of-reclaimer.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/mglru: lift the forced protection of reclaimer
has been added to the -mm mm-new branch.  Its filename is
     mm-mglru-lift-the-forced-protection-of-reclaimer.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mglru-lift-the-forced-protection-of-reclaimer.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 <kasong@tencent.com>
Subject: mm/mglru: lift the forced protection of reclaimer
Date: Thu, 12 Nov 2026 07:25:51 +0800

When a type is drained down to MIN_NR_GENS gens, the reclaimer will
fail to make any progress on that type without aging.

However aging comes with a cost, and it will take time to finish. Even
if we do aging more proactively, when under great pressure, gens will
still be drained very fast so the new gens produced by aging can't catch
up with the consumption. With MIN_NR_GENS setting blocking the reclaim,
we will run into OOM. This OOM can be reproduced by the previous test
program [1] used for fixing another aging problem [2]. When running in
great pressure, certain workloads will keep seeing premature OOM with
MGLRU, and never see any OOM with classical LRU.

This might get worse if we implement async aging, or avoid hotness
inversion in aging, as they will slow down the aging process even more.

To fix this, let's first drop the get_nr_gens() == MIN_NR_GENS early
return and let the raised-priority scan bound cover max_seq itself.

Mapped folios will still survive as the rmap PTE check will keep them.
Unmapped cache folios may thrashing more when under great pressure,
which is really a problem that should be fixed with refault distance
and frequecy aware promotoin instead [3].

Link: https://lore.kernel.org/20261111232603.9934-5-baohua@kernel.org
Link: https://github.com/ryncsn/emm-test-project/tree/master/file-anon-mix-pressure [1]
Link: https://lore.kernel.org/linux-mm/20260428-mglru-reclaim-v7-0-02fabb92dc43@tencent.com/ [2]
Link: https://lore.kernel.org/linux-mm/20260502-mglru-fg-v1-0-913619b014d9@tencent.com/ [3]
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <hui.zhu@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: wang lian <lianux.mm@gmail.com>
Cc: <liuxiang.277@bytedance.com>
Cc: <lyugaofei@xiaomi.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Ridong Chen <ridong.chen@linux.dev>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: wangzicheng <wangzicheng@honor.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Bo Zhang <zhangbo56@xiaomi.com>
Cc: Hui Zhu <zhuhui@kylinos.cn>
Cc: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
Cc: Kunwu Chan <chentao@kylinos.cn>
---

 mm/vmscan.c |   34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

--- a/mm/vmscan.c~mm-mglru-lift-the-forced-protection-of-reclaimer
+++ a/mm/vmscan.c
@@ -4679,7 +4679,7 @@ void lru_gen_reparent_memcg(struct mem_c
  ******************************************************************************/
 
 static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_control *sc,
-		       int min_gen, int tier_idx)
+		       int min_gen, int max_gen, int tier_idx)
 {
 	int gen = folio_lru_gen(folio);
 	int type = folio_is_file_lru(folio);
@@ -4696,12 +4696,22 @@ static bool sort_folio(struct lruvec *lr
 	if (!folio_evictable(folio))
 		return false;
 
-	/* promoted */
+	/* promoted, or moved due to reparenting. */
 	if (gen != min_gen) {
 		list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
 		return true;
 	}
 
+	/*
+	 * We are reclaiming the last gen left. Folios in this gen can no
+	 * longer be promoted or protected, so just isolate them. The one
+	 * exception is lru_gen_reparent_memcg(), which is handled above.
+	 */
+	if (min_gen == max_gen) {
+		VM_WARN_ON_ONCE(zone > sc->reclaim_idx);
+		return false;
+	}
+
 	/* protected */
 	if (tier > tier_idx || refs + workingset == BIT(LRU_REFS_WIDTH) + 1) {
 		gen = folio_inc_gen(lruvec, folio, min_gen);
@@ -4756,15 +4766,16 @@ static int scan_folios(unsigned long nr_
 		       struct list_head *list, int *isolatedp)
 {
 	int i;
-	int gen;
 	enum node_stat_item item;
 	int sorted = 0;
 	int scanned = 0;
 	int isolated = 0;
 	int skipped = 0;
+	int gen, max_gen;
 	unsigned long min_seq;
 	unsigned long remaining = nr_to_scan;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	DEFINE_MAX_SEQ(lruvec);
 
 	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
 	VM_WARN_ON_ONCE(!list_empty(list));
@@ -4777,18 +4788,24 @@ static int scan_folios(unsigned long nr_
 	if (sc->priority == DEF_PRIORITY)
 		min_seq = lrugen->min_seq[type];
 	else
-		min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq - MIN_NR_GENS + 1);
-
-	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
-		return 0;
+		min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq);
 
 	gen = lru_gen_from_seq(min_seq);
+	max_gen = lru_gen_from_seq(max_seq);
+
 	for (i = MAX_NR_ZONES; i > 0; i--) {
 		LIST_HEAD(moved);
 		int skipped_zone = 0;
 		int zone = (sc->reclaim_idx + i) % MAX_NR_ZONES;
 		struct list_head *head = &lrugen->folios[gen][type][zone];
 
+		/*
+		 * If there is only one gen left, rotating the unreclaimable
+		 * zone become meaningless, just skip them.
+		 */
+		if (gen == max_gen && zone > sc->reclaim_idx)
+			continue;
+
 		while (!list_empty(head)) {
 			struct folio *folio = lru_to_folio(head);
 			int delta = folio_nr_pages(folio);
@@ -4799,8 +4816,7 @@ static int scan_folios(unsigned long nr_
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
 			scanned += delta;
-
-			if (sort_folio(lruvec, folio, sc, gen, tier))
+			if (sort_folio(lruvec, folio, sc, gen, max_gen, tier))
 				sorted += delta;
 			else if (isolate_folio(lruvec, folio, sc)) {
 				list_add(&folio->lru, list);
_

Patches currently in -mm which might be from kasong@tencent.com are

mm-mglru-report-evictable-sizes-per-type.patch
mm-mglru-extract-lruvec_populated_min_seq-helper.patch
mm-mglru-reclaim-from-the-oldest-populated-gen.patch
mm-mglru-lift-the-forced-protection-of-reclaimer.patch
mm-mglru-skip-zones-above-reclaim_idx-instead-of-rotating-them.patch
mm-mglru-return-for-unavailable-anon-reclaim-earlier.patch
mm-mglru-balance-anon-and-file-scan-budget-by-refault-rate.patch
mm-mglru-lift-youngest-gen-protection-for-higher-priority.patch