[merged mm-stable] mm-mglru-separate-folio-generation-update-from-lru-accounting.patch removed from -mm tree

[merged mm-stable] mm-mglru-separate-folio-generation-update-from-lru-accounting.patch removed from -mm tree

[Andrew Morton]

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The quilt patch titled
     Subject: mm/mglru: separate folio generation update from LRU accounting
has been removed from the -mm tree.  Its filename was
     mm-mglru-separate-folio-generation-update-from-lru-accounting.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: "Barry Song (Xiaomi)" <baohua@kernel.org>
Subject: mm/mglru: separate folio generation update from LRU accounting
Date: Wed, 2 Sep 2026 07:24:15 +0800

Patch series "mm/mglru: speed up inc_min_seq() and fix cold/hot
inversions", v3.


This is an aging speedup series split out from the MGLRU swappiness series
[1], with the inc_min_seq changes separated to make them easier to review.

Currently, inc_min_seq() has two primary issues that affect both
performance and folio hotness assessment:

1. It processes each folio one by one, while many operations can be
   batched or skipped.  For example, a batch of folios can be moved
   together from the oldest generation to the second-oldest generation,
   and the associated counting can also be done in batches.

2. It may cause potential cold/hot inversion by placing promoted
   folios (which have been scanned and found to have young PTEs) behind
   non-promoted folios.  A similar inversion can also occur among
   non-promoted folios, as tail folios from the oldest generation are
   placed before head folios when moving them to the second-oldest
   generation.

This series tries to batch operations as much as possible and fix the
potential cold/hot inversion by keeping promoted folios ahead of
non-promoted folios, while also preserving the order of non-promoted
folios when moving them from the oldest generation to the second-oldest
generation.

Minor issue: inc_min_seq() also counts protected folios improperly, as
promoted folios should be skipped, as in sort_folio().

We need a stable workload with a stable number of folios to measure aging
and evaluate the speedup in inc_min_seq().  So I asked ChatGPT to generate
the microbenchmark below.  It ages an LRU vec containing 512 MB of memory
100 times:

 #define _GNU_SOURCE

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
 #include <time.h>
 #include <sys/mman.h>

 #define SIZE		(512UL * 1024 * 1024)
 #define LRU_GEN		"/sys/kernel/debug/lru_gen"
 #define TARGET_CGROUP	"/system.slice/agetest.scope"
 #define START_GEN	3
 #define END_GEN	103

 static long long nsec_diff(const struct timespec *start,
 			   const struct timespec *end)
 {
 	return (end->tv_sec - start->tv_sec) * 1000000000LL +
 	       (end->tv_nsec - start->tv_nsec);
 }

 static int find_memcg_id(void)
 {
 	FILE *fp;
 	char line[4096];
 	int memcg_id;

 	fp = fopen(LRU_GEN, "r");
 	if (!fp) {
 		perror("fopen lru_gen");
 		return -1;
 	}

 	while (fgets(line, sizeof(line), fp)) {
 		char *p;

 		if (strncmp(line, "memcg ", 6))
 			continue;

 		p = line + 6;

 		if (sscanf(p, "%d", &memcg_id) != 1)
 			continue;

 		/*
 		 * The memcg path follows the numeric ID.
 		 */
 		p = strchr(p, ' ');
 		if (!p)
 			continue;

 		if (strstr(p, TARGET_CGROUP)) {
 			fclose(fp);
 			return memcg_id;
 		}
 	}

 	fclose(fp);

 	fprintf(stderr, "Cannot find %s\n", TARGET_CGROUP);
 	return -1;
 }

 int main(void)
 {
 	void *addr;
 	int memcg_id;
 	int fd;
 	long long total_ns = 0;

 	/*
 	 * mmap 512 MB and touch every page.
 	 */
 	addr = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
 		    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 	if (addr == MAP_FAILED) {
 		perror("mmap");
 		return 1;
 	}

 	memset(addr, 0x55, SIZE);

 	printf("mmap: %p, size: %lu MB\n",
 	       addr, SIZE / 1024 / 1024);

 	/*
 	 * Find the memcg ID automatically.
 	 */
 	memcg_id = find_memcg_id();
 	if (memcg_id < 0)
 		return 1;

 	printf("memcg: %d (%s)\n", memcg_id, TARGET_CGROUP);
 	printf("aging generation %d -> %d\n",
 	       START_GEN, END_GEN);

 	fd = open(LRU_GEN, O_WRONLY);
 	if (fd < 0) {
 		perror("open lru_gen");
 		return 1;
 	}

 	for (int gen = START_GEN; gen <= END_GEN; gen++) {
 		char buf[128];
 		int len;
 		struct timespec start, end;
 		long long ns;

 		len = snprintf(buf, sizeof(buf),
 			       "+ %d 0 %d\n", memcg_id, gen);

 		clock_gettime(CLOCK_MONOTONIC, &start);

 		if (write(fd, buf, len) != len) {
 			perror("write lru_gen");
 			close(fd);
 			return 1;
 		}

 		clock_gettime(CLOCK_MONOTONIC, &end);

 		ns = nsec_diff(&start, &end);
 		total_ns += ns;

 		printf("gen %3d: %8.3f ms\n",
 		       gen, ns / 1000000.0);
 		fflush(stdout);
 	}

 	close(fd);

 	printf("\nTotal:   %.3f ms\n",
 	       total_ns / 1000000.0);
 	printf("Average: %.3f ms\n",
 	       total_ns / (double)(END_GEN - START_GEN + 1) /
 	       1000000.0);

 	while (1)
 		sleep(1);

 	return 0;
 }

Run the above microbenchmark with:
systemd-run --scope --unit=agetest -p MemoryMax=1024M ./agetest

I’m seeing a significant speedup in inc_min_seq() on my x86 PC:

W/o patch:

Running scope as unit: agetest.scope
mmap: 0x72c1b5a00000, size: 512 MB
memcg: 12673 (/system.slice/agetest.scope)
aging generation 3 -> 103
gen   3:    7.433 ms
gen   4:    0.949 ms
gen   5:    2.535 ms
gen   6:    5.043 ms
gen   7:    5.041 ms
gen   8:    5.027 ms
...
gen 100:    5.035 ms
gen 101:    5.011 ms
gen 102:    5.029 ms
gen 103:    5.056 ms

Total:   503.946 ms
Average: 4.990 ms

W/ patch:

Running scope as unit: agetest.scope
mmap: 0x775ec5200000, size: 512 MB
memcg: 12717 (/system.slice/agetest.scope)
aging generation 3 -> 103
gen   3:    7.558 ms
gen   4:    0.916 ms
gen   5:    2.277 ms
gen   6:    2.328 ms
…
gen 100:    2.303 ms
gen 101:    2.297 ms
gen 102:    2.305 ms
gen 103:    2.312 ms

Total:   236.635 ms
Average: 2.343 ms

The average aging time drops from 4.990 ms to 2.343 ms!

Thanks, Xueyuan, for testing this on ARM[2].  It actually shows an even
larger improvement.

Xueyuan tested this series on his arm64 machine (24 cores, 4K base pages)
and reproduced the improvement:

THP=never (PTE):
  baseline: 7.644 ms
  patched:  2.964 ms (-61.2%)

THP=always (PMD):
  baseline: 0.0373 ms
  patched:  0.0292 ms (-21.8%)


This patch (of 7):


folio_inc_gen() currently updates both the folio's generation and the LRU
size accounting.  This makes it difficult to batch the LRU size updates
when moving multiple folios.

Extract the generation update into __folio_inc_gen(), which only updates
the folio's generation and reports whether the generation was actually
increased.  Keep folio_inc_gen() as the wrapper that performs the LRU size
accounting when needed.

This separates the per-folio generation update from LRU accounting and
allows the latter to be batched by subsequent changes.

Link: https://lore.kernel.org/20260901232421.40157-1-baohua@kernel.org
Link: https://lore.kernel.org/20260901232421.40157-2-baohua@kernel.org
Link: https://lore.kernel.org/linux-mm/20260812121658.69965-1-baohua@kernel.org/ [1]
Link: https://lore.kernel.org/linux-mm/20260827035416.3012015-1-xueyuan.chen21@gmail.com/ [2]
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
Reviewed-by: Lian Wang <lianux.mm@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Stevens <stevensd@chromium.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Kunwu Chan <kunwu.chan@gmail.com>
Cc: Ridong Chen <ridong.chen@linux.dev>
---

 mm/vmscan.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

--- a/mm/vmscan.c~mm-mglru-separate-folio-generation-update-from-lru-accounting
+++ a/mm/vmscan.c
@@ -3300,21 +3300,21 @@ static int folio_update_gen(struct folio
 	return ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
 }
 
-/* protect pages accessed multiple times through file descriptors */
-static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
+static int __folio_inc_gen(struct folio *folio, int old_gen, bool *increased)
 {
-	int type = folio_is_file_lru(folio);
-	struct lru_gen_folio *lrugen = &lruvec->lrugen;
-	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
 	unsigned long new_flags, old_flags = READ_ONCE(folio->flags.f);
+	int new_gen;
 
 	VM_WARN_ON_ONCE_FOLIO(!(old_flags & LRU_GEN_MASK), folio);
 
 	do {
 		new_gen = ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
 		/* folio_update_gen() has promoted this page? */
-		if (new_gen >= 0 && new_gen != old_gen)
+		if (new_gen >= 0 && new_gen != old_gen) {
+			if (increased)
+				*increased = false;
 			return new_gen;
+		}
 
 		new_gen = (old_gen + 1) % MAX_NR_GENS;
 
@@ -3322,8 +3322,22 @@ static int folio_inc_gen(struct lruvec *
 		new_flags |= (new_gen + 1UL) << LRU_GEN_PGOFF;
 	} while (!try_cmpxchg(&folio->flags.f, &old_flags, new_flags));
 
-	lru_gen_update_size(lruvec, folio, old_gen, new_gen);
+	if (increased)
+		*increased = true;
+	return new_gen;
+}
+
+/* protect pages accessed multiple times through file descriptors */
+static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
+{
+	int type = folio_is_file_lru(folio);
+	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
+	bool gen_increased;
 
+	new_gen = __folio_inc_gen(folio, old_gen, &gen_increased);
+	if (gen_increased)
+		lru_gen_update_size(lruvec, folio, old_gen, new_gen);
 	return new_gen;
 }
 
_

Patches currently in -mm which might be from baohua@kernel.org are

mm-mglru-restore-accidentally-removed-seq-max_seq-check.patch