[RFC PATCH v2 0/5] mm: mglru: fix swappiness behavior

[RFC PATCH v2 0/5] mm: mglru: fix swappiness behavior

[Barry Song (Xiaomi)]

RFC v2:
 - Quickly address a few issues raised in the Sashiko comments so
   reviewers can ignore v1 and review a cleaner version instead.
   https://sashiko.dev/#/patchset/20260726012946.18684-1-baohua@kernel.org
Thanks, Sashiko!

The active/inactive LRU respects swappiness well. Anonymous page
scanning and reclamation increase roughly linearly with
swappiness, while file page scanning and reclamation decrease
accordingly.
For example, when swappiness reaches 200, both pgsteal_file and
pgscan_file drop to zero while building the kernel in a 1 GB
memcg. In contrast, MGLRU shows almost no change across different
swappiness values.

                   pgsteal_file

Swappiness    LRU         MGLRU
--------------------------------
1             10567455      763612
36              990706      480205
71              688170      415848
106             446294      386164
141             286307      359196
176             201733      351686
200                  0      330093


                   pgsteal_anon

Swappiness    LRU         MGLRU
--------------------------------
1              4410548     2726362
36             2465268     2762859
71             2677908     2885124
106            2737227     2841796
141            2984276     3035015
176            3381338     2938302
200           13116359     3113499


                   pgscan_file

Swappiness    LRU         MGLRU
--------------------------------
1             17997223      923094
36             1325674      539571
71              852345      464222
106             538207      464477
141             357253      412277
176             217536      399446
200                  0      375902


                   pgscan_anon

Swappiness    LRU         MGLRU
--------------------------------
1             31639423     5987136
36            23441521     5753224
71            26067110     6101780
106           25619448     5782919
141           26842088     6234264
176           29200021     5980292
200           62193924     6413125

This patchset respects the type selected by positive_ctrl_err(),
which uses swappiness as its gain. It does so by:

1. Avoiding premature fallback to the other type. Only fall back
   when reclaim is running at high priority.

2. Running aging when the preferred type has few or no
   reclaimable folios, so more folios of that type become
   reclaimable.

With this patchset, swappiness starts to behave similarly to the
active/inactive LRU.

                         pgsteal_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             10567455      763612        2276977
36              990706      480205         485771
71              688170      415848         425893
106             446294      386164         391528
141             286307      359196         371844
176             201733      351686         322634
200                  0      330093           1343


                         pgsteal_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1              4410548     2726362        2212243
36             2465268     2762859        3036395
71             2677908     2885124        3111459
106            2737227     2841796        3021429
141            2984276     3035015        3341110
176            3381338     2938302        3232294
200           13116359     3113499       14144584


                         pgscan_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             17997223      923094        3243420
36             1325674      539571         595233
71              852345      464222         523441
106             538207      464477         457391
141             357253      412277         434981
176             217536      399446         369302
200                  0      375902           1344


                         pgscan_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             31639423     5987136        4136386
36            23441521     5753224        5489636
71            26067110     6101780        5699571
106           25619448     5782919        5910594
141           26842088     6234264        6511215
176           29200021     5980292        6286023
200           62193924     6413125       22582174

Another possible approach is to decouple anonymous and file-backed
aging by maintaining separate max_seq values for each type. This
allows anonymous and file-backed memory to age and be reclaimed
independently, enabling the swappiness-preferred type to be reclaimed
more aggressively while allowing the other type to lag behind. This
approach has already been adopted by projects such as CachyOS [1] and
Chromium [2].

However, this approach requires substantial changes to MGLRU and
fundamentally alters its design by breaking the shared aging timeline
between anonymous and file-backed memory. This timeline is the
foundation for mechanisms such as the PID controller and the
min_ttl_ms thrashing protection.

That is why this patchset aims to fix the swappiness behavior
without fundamentally changing MGLRU's design, with minimal changes.

[1] https://github.com/firelzrd/re-swappiness
[2] https://chromium.googlesource.com/chromiumos/third_party/kernel/+log/929932351492d01f0aee37a0ac3be8c7bd88f80d

Barry Song (Xiaomi) (4):
  mm: mglru: avoid scanning empty generations in scan_folios()
  mm: mglru: only fall back when reclaim is running at high priority
  mm: mglru: prevent min_seq[type] from pointing to an empty generation
  mm: mglru: run aging if the preferred type has no folios in
    reclaimable gens

lyugaofei (1):
  mm: mglru: run aging when pages are severely imbalanced across gens

 include/linux/mmzone.h |  6 ++--
 mm/vmscan.c            | 66 ++++++++++++++++++++++++++++++++----------
 2 files changed, 53 insertions(+), 19 deletions(-)

-- 
2.39.3 (Apple Git-146)

[RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

[Barry Song (Xiaomi)]

Commit 16b475d2ac3c ("mm/mglru: avoid reclaim type fall back when
isolation makes no progress") only falls back to the other type when
scanned == 0. However, I have frequently observed cases where
scanned > 0, but the older reclaimable generation becomes empty
after the first scan_folios(). As a result, the second
scan_folios() for the same type performs a redundant scan over an
empty generation.

We can avoid this by checking whether the reclaimable generation has
become empty when scanned < nr_to_scan and we still have fewer than
MIN_LRU_BATCH isolated folios after scan_folios().

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 566c4e837c7d..babbce4bbfe8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4852,11 +4852,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			break;
 		}
 		/*
-		 * If scanned > 0 and isolated == 0, avoid falling back to the
-		 * other type, as this type remains sufficient. Falling back
-		 * too readily can disrupt the positive_ctrl_err() bias.
+		 * If scanned >= nr_to_scan or isolated >= MIN_LRU_BATCH,
+		 * avoid falling back to the other type. The preferred
+		 * type is still reclaimable; otherwise, it would have
+		 * already run out of reclaimable generations. Falling
+		 * back too readily can disrupt the positive_ctrl_err()
+		 * bias.
 		 */
-		if (!scanned)
+		if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
 			type = !type;
 	}
 
-- 
2.39.3 (Apple Git-146)

[RFC PATCH v2 2/5] mm: mglru: only fall back when reclaim is running at high priority

[Barry Song (Xiaomi)]

Respect the type selected by positive_ctrl_err(), where swappiness
controls the relative weights of SP and PV. Falling back too readily
undermines that bias. Only fall back after making a sufficient effort
to reclaim from the preferred type.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index babbce4bbfe8..4a387cc4145a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4857,10 +4857,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 		 * type is still reclaimable; otherwise, it would have
 		 * already run out of reclaimable generations. Falling
 		 * back too readily can disrupt the positive_ctrl_err()
-		 * bias.
+		 * bias. Also, only fall back when reclaim is running at
+		 * a high priority.
 		 */
-		if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
+		if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
+			if (sc->priority > 2)
+				break;
 			type = !type;
+		}
 	}
 
 	return total_scanned;
-- 
2.39.3 (Apple Git-146)

[RFC PATCH v2 3/5] mm: mglru: prevent min_seq[type] from pointing to an empty generation

[Barry Song (Xiaomi)]

In try_to_inc_min_seq(), min_seq[LRU_GEN_ANON] and
min_seq[LRU_GEN_FILE] can be fixed up to point to empty generations
in order to keep their gap within one. As a result, scan_folios()
may repeatedly scan empty generations. This is confusing, as I
observed scan_folios() returning 0 even though the following check in
scan_folios() doesn't take effect:

        if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
                return 0;

There is no need to adjust min_seq[], since the reclaim logic already
triggers aging when the number of generations reaches
MIN_NR_GENS, and reclaim never reduces it below MIN_NR_GENS.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 include/linux/mmzone.h |  6 ++----
 mm/vmscan.c            | 10 ----------
 2 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index a26c8b855222..233d2006a541 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -552,10 +552,8 @@ enum {
  * The youngest generation number is stored in max_seq for both anon and file
  * types as they are aged on an equal footing. The oldest generation numbers are
  * stored in min_seq[] separately for anon and file types so that they can be
- * incremented independently. Ideally min_seq[] are kept in sync when both anon
- * and file types are evictable. However, to adapt to situations like extreme
- * swappiness, they are allowed to be out of sync by at most
- * MAX_NR_GENS-MIN_NR_GENS-1.
+ * incremented independently. For both file and anonymous memory, the minimum
+ * generation must be at least MIN_NR_GENS.
  *
  * The number of pages in each generation is eventually consistent and therefore
  * can be transiently negative when reset_batch_size() is pending.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4a387cc4145a..d6bd64b4dced 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3972,16 +3972,6 @@ static void try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 	if (!seq_inc_flag)
 		return;
 
-	/* see the comment on lru_gen_folio */
-	if (swappiness && swappiness <= MAX_SWAPPINESS) {
-		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;
-
-		if (min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] < seq)
-			min_seq[LRU_GEN_ANON] = seq;
-		else if (min_seq[LRU_GEN_FILE] > seq && min_seq[LRU_GEN_ANON] < seq)
-			min_seq[LRU_GEN_FILE] = seq;
-	}
-
 	for_each_evictable_type(type, swappiness) {
 		if (min_seq[type] <= lrugen->min_seq[type])
 			continue;
-- 
2.39.3 (Apple Git-146)

[RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Barry Song (Xiaomi)]

From: lyugaofei <lyugaofei@xiaomi.com>

This partially restores the reclaim behavior introduced in Yu
Zhao's initial MGLRU commit, ac35a4902370 ("mm: multi-gen LRU:
minimal implementation"):
	/*
	 * It's also ideal to spread pages out evenly, i.e., 1/(MIN_NR_GENS+1)
	 * of the total number of pages for each generation. A reasonable range
	 * for this average portion is [1/MIN_NR_GENS, 1/(MIN_NR_GENS+2)]. The
	 * aging cares about the upper bound of hot pages, while the eviction
	 * cares about the lower bound of cold pages.
	 */
	if (young * MIN_NR_GENS > total)
		return true;
	if (old * (MIN_NR_GENS + 2) < total)
		return true;

But with a stricter condition: the younger generations must contain
at least four times as many folios as the older generations. This
allows aging to keep folios of the preferred type spread across the
reclaimable generations.

Signed-off-by: lyugaofei <lyugaofei@xiaomi.com>
Co-developed-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d6bd64b4dced..7c13dedb0b1f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4956,9 +4956,40 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	return scanned;
 }
 
+static bool lru_gen_imbalanced(struct lruvec *lruvec, int type,
+		unsigned long max_seq, unsigned long min_seq,
+		int swappiness)
+{
+	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	unsigned long young = 0, old = 0, seq;
+
+	/*
+	 * reclaim is forced to a single type in those cases, so there is
+	 * no need to consider the swappiness bias
+	 */
+	if (swappiness == MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
+		return false;
+
+	for (seq = min_seq; seq <= max_seq; seq++) {
+		int gen = lru_gen_from_seq(seq);
+		unsigned long size = 0;
+		int zone;
+
+		for (zone = 0; zone < MAX_NR_ZONES; zone++)
+			size += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
+
+		if (seq + MIN_NR_GENS > max_seq)
+			young += size;
+		else
+			old += size;
+	}
+	return young > old * 4;
+}
+
 static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
 			     struct scan_control *sc, int swappiness)
 {
+	int type = get_type_to_scan(lruvec, swappiness);
 	DEFINE_MIN_SEQ(lruvec);
 
 	/* have to run aging, since eviction is not possible anymore */
@@ -4970,7 +5001,11 @@ static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
 		return false;
 
 	/* better to run aging even though eviction is still possible */
-	return evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq;
+	if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq)
+		return true;
+
+	/* Run aging if the preferred type is severely imbalanced across gens */
+	return lru_gen_imbalanced(lruvec, type, max_seq, min_seq[type], swappiness);
 }
 
 static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
-- 
2.39.3 (Apple Git-146)

[RFC PATCH v2 5/5] mm: mglru: run aging if the preferred type has no folios in reclaimable gens

[Barry Song (Xiaomi)]

Respect the type selected by positive_ctrl_err(). If there are no
reclaimable gens left for that type, run aging.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7c13dedb0b1f..d63322cdb1bc 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4996,6 +4996,10 @@ static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
 	if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
 		return true;
 
+	/* run aging if the preferred type is exhausted */
+	if (min_seq[type] + MIN_NR_GENS > max_seq)
+		return true;
+
 	/* try to avoid aging, do gentle reclaim at the default priority */
 	if (sc->priority == DEF_PRIORITY)
 		return false;
-- 
2.39.3 (Apple Git-146)

Re: [RFC PATCH v2 2/5] mm: mglru: only fall back when reclaim is running at high priority

[Barry Song]

On Sun, Jul 26, 2026 at 8:21 PM Barry Song (Xiaomi) <baohua@kernel.org> wrote:
>
> Respect the type selected by positive_ctrl_err(), where swappiness
> controls the relative weights of SP and PV. Falling back too readily
> undermines that bias. Only fall back after making a sufficient effort
> to reclaim from the preferred type.
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/vmscan.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index babbce4bbfe8..4a387cc4145a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4857,10 +4857,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>                  * type is still reclaimable; otherwise, it would have
>                  * already run out of reclaimable generations. Falling
>                  * back too readily can disrupt the positive_ctrl_err()
> -                * bias.
> +                * bias. Also, only fall back when reclaim is running at
> +                * a high priority.
>                  */
> -               if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
> +               if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
> +                       if (sc->priority > 2)
> +                               break;
>                         type = !type;
> +               }

https://sashiko.dev/#/patchset/20260726122123.7614-1-baohua%40kernel.org

"> diff --git a/mm/vmscan.c b/mm/vmscan.c
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4867,10 +4867,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
[ ... ]
> - if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
> + if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
> + if (sc->priority > 2)
> + break;
Does this logic artificially drop the scan priority and cause false memory
pressure events when the preferred LRU type is empty?
By breaking the isolation loop instead of falling back to the populated
LRU list, the direct reclaim path might falsely conclude that no pages can
be isolated at the current priority. This forces the VM to artificially drop
the scanning priority from the default priority down to 2.
This rapid priority drop could have two severe consequences. First, it
aggressively invokes shrink_slab() at artificially high priorities, causing
unnecessary eviction of active slab objects (like dentries and inodes)
which results in extreme latency spikes.
Second, when sc->priority drops to 3, vmpressure_prio() triggers a critical
memory pressure event. Userspace daemons (like lmkd or oomd) might respond
by killing background apps, falsely assuming the system is completely out
of memory, even though the non-preferred LRU list could be full of clean,
evictable pages.
> type = !type;
> + }
> }
>
> return total_scanned;"

I can change the priority to "DEF_PRIORITY - 2":

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d63322cdb1bc..12eba6216fde 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4851,7 +4851,7 @@ static int isolate_folios(unsigned long
nr_to_scan, struct lruvec *lruvec,
                 * a high priority.
                 */
                if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
-                       if (sc->priority > 2)
+                       if (sc->priority > DEF_PRIORITY - 2)
                                break;
                        type = !type;
                }

And the result is still fine:

                         pgsteal_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             10567455      763612        2191652
36              990706      480205         414483
71              688170      415848         386966
106             446294      386164         354976
141             286307      359196         348493
176             201733      351686         297113
200                  0      330093           2544


                         pgsteal_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1              4410548     2726362        3511241
36             2465268     2762859        3027445
71             2677908     2885124        3359352
106            2737227     2841796        3224762
141            2984276     3035015        3221214
176            3381338     2938302        3446206
200           13116359     3113499       14700981


                         pgscan_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             17997223      923094        3178493
36             1325674      539571         517029
71              852345      464222         460869
106             538207      464477         411314
141             357253      412277         402429
176             217536      399446         333375
200                  0      375902           3083


                         pgscan_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             31639423     5987136        6184980
36            23441521     5753224        5727338
71            26067110     6101780        5932826
106           25619448     5782919        6105922
141           26842088     6234264        6147981
176           29200021     5980292        6707570
200           62193924     6413125       23288983



If I drop this patch entirely, swappiness still functions:


                         pgsteal_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             10567455      763612        2118900
36              990706      480205         426026
71              688170      415848         396647
106             446294      386164         376780
141             286307      359196         334243
176             201733      351686         287544
200                  0      330093            309


                         pgsteal_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1              4410548     2726362        3579570
36             2465268     2762859        3300899
71             2677908     2885124        3183903
106            2737227     2841796        3419872
141            2984276     3035015        3121920
176            3381338     2938302        3198154
200           13116359     3113499       14348490


                         pgscan_file

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             17997223      923094        2898754
36             1325674      539571         509825
71              852345      464222         470746
106             538207      464477         434145
141             357253      412277         378242
176             217536      399446         322824
200                  0      375902            694


                         pgscan_anon

Swappiness      LRU        MGLRU      MGLRU+Patch
-------------------------------------------------
1             31639423     5987136        6594803
36            23441521     5753224        5979158
71            26067110     6101780        6200677
106           25619448     5782919        6480037
141           26842088     6234264        6106924
176           29200021     5980292        6553534
200           62193924     6413125       22763932

So I guess the other aging patches have a larger impact on
improving swappiness behavior than this one.

Best Regards
Barry

Re: [RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

[Barry Song]

On Sun, Jul 26, 2026 at 8:21 PM Barry Song (Xiaomi) <baohua@kernel.org> wrote:
>
> Commit 16b475d2ac3c ("mm/mglru: avoid reclaim type fall back when
> isolation makes no progress") only falls back to the other type when
> scanned == 0. However, I have frequently observed cases where
> scanned > 0, but the older reclaimable generation becomes empty
> after the first scan_folios(). As a result, the second
> scan_folios() for the same type performs a redundant scan over an
> empty generation.
>
> We can avoid this by checking whether the reclaimable generation has
> become empty when scanned < nr_to_scan and we still have fewer than
> MIN_LRU_BATCH isolated folios after scan_folios().
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/vmscan.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 566c4e837c7d..babbce4bbfe8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4852,11 +4852,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>                         break;
>                 }
>                 /*
> -                * If scanned > 0 and isolated == 0, avoid falling back to the
> -                * other type, as this type remains sufficient. Falling back
> -                * too readily can disrupt the positive_ctrl_err() bias.
> +                * If scanned >= nr_to_scan or isolated >= MIN_LRU_BATCH,
> +                * avoid falling back to the other type. The preferred
> +                * type is still reclaimable; otherwise, it would have
> +                * already run out of reclaimable generations. Falling
> +                * back too readily can disrupt the positive_ctrl_err()
> +                * bias.
>                  */
> -               if (!scanned)
> +               if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)

https://sashiko.dev/#/patchset/20260726122123.7614-1-baohua%40kernel.org

"
> - if (!scanned)
> + if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
> type = !type;
> }
Does the check for *isolated < MIN_LRU_BATCH have any effect here?
Looking earlier in isolate_folios(), there is an if (*isolated) check that
breaks out of the loop:
if (*isolated)
break;
Since execution only reaches this point if *isolated is exactly 0, isn't
*isolated < MIN_LRU_BATCH unconditionally true?
Additionally, will this logic work correctly when large folios are involved?
If scan_folios() encounters an un-isolatable large folio (for example, an
order-9 THP with 512 pages), the scanned variable is incremented by the base
page count (512).
Since nr_to_scan is capped at MIN_LRU_BATCH (64) by the caller evict_folios(),
scanned < nr_to_scan (512 < 64) will evaluate to false even if the generation
is completely empty.
Does this prevent the fallback and inadvertently defeat the optimization on
systems utilizing large folios?
"

Right. What I actually need is a way to tell when scan_folios()
returns after the current generation's LRU lists are empty, but I
haven't found a good approach. Checking scanned < nr_to_scan works
well for small folios, but not for large folios.

Maybe we could add an argument to scan_folios(), like this:

@@ -4707,7 +4711,8 @@ static bool isolate_folio(struct lruvec *lruvec,
struct folio *folio, struct sca

 static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
                       struct scan_control *sc, int type, int tier,
-                      struct list_head *list, int *isolatedp)
+                      struct list_head *list, int *isolatedp,
+                      bool *exhausted)
 {
        int i;
        int gen;
@@ -4776,6 +4781,11 @@ static int scan_folios(unsigned long
nr_to_scan, struct lruvec *lruvec,
                                scanned, skipped, isolated,
                                type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);

+       /*
+        * This scan exhausted the current generation before reaching the
+        * scan target or accumulating enough isolated folios.
+        */
+       *exhausted = remaining > 0 && isolated < MIN_LRU_BATCH;
        *isolatedp = isolated;
        return scanned;
 }

Then another question comes up: suppose we have four generations, and
scan_folios() exhausts the oldest one while the second-oldest still
contains reclaimable folios. The current implementation doesn't move
on to scan the second-oldest generation. Instead, it returns,
leaving that generation with no chance to be scanned at the current
sc->priority. That doesn't seem right. Let me investigate this as
well.

Thanks
Barry

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Zicheng Wang]

Hi Barry, lyugaofei,

The cover letter results look good.

This change looks like it resolves the cross-type skew from 798c0330c2ca
[1] (if I understand correctly).

But:

  removed (798c0330c2ca):  old * 4 < total
  4/5:                     old * 4 < young

Can a single type also skew, and does changing total to young have any
downside?

Nit: the old form derived the constant from (MIN_NR_GENS + 2).

[1] https://lore.kernel.org/all/20241231043538.4075764-4-yuzhao@google.com/

Best,
Zicheng

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Barry Song]

On Tue, Jul 28, 2026 at 11:46 AM Zicheng Wang <wangzicheng@honor.com> wrote:
>
> Hi Barry, lyugaofei,
>
> The cover letter results look good.
>
> This change looks like it resolves the cross-type skew from 798c0330c2ca
> [1] (if I understand correctly).

Hi Zicheng,

I guess it somehow moves MGLRU back toward its original design
of maintaining some balance across generations.

>
> But:
>
>   removed (798c0330c2ca):  old * 4 < total
>   4/5:                     old * 4 < young
>
> Can a single type also skew, and does changing total to young have any
> downside?

The original code is a bit odd, as it compares a single young
generation and a single old generation against the total,
rather than comparing all young generations with all old
generations.
 if (seq == max_seq)
         young += size;
 else if (seq + MIN_NR_GENS == max_seq)
        old += size;

Also, for the swappiness bias, we care more about the preferred
type than the other type. So comparing a single type, rather
than the sum of both types, makes more sense.

>
> Nit: the old form derived the constant from (MIN_NR_GENS + 2).

By comparing all young generations against all old
generations, we no longer depend on the exact value of
MIN_NR_GENS. The idea is to emulate inactive_is_low().

>
> [1] https://lore.kernel.org/all/20241231043538.4075764-4-yuzhao@google.com/
>

Thanks
Barry

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Zicheng Wang]

Hi Barry,

> The original code is a bit odd, as it compares a single young
> generation and a single old generation against the total, rather than
> comparing all young generations with all old generations.
>
> By comparing all young generations against all old generations, we no
> longer depend on the exact value of MIN_NR_GENS. The idea is to
> emulate inactive_is_low().

Makes sense, the inactive_is_low() framing fits.

One gap: inactive_is_low()'s ratio is dynamic calculated. 
int_sqrt(10*1024) = 101 (ages only when inactive drops below ~1%),
while patch 4/5's fixed 4 ages once old falls under 20% of young.
It seems there will be considerable cost on big memcgs, or should
the ratio scale with size?

Looking forward to the perf data.

Thanks,
Zicheng

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Barry Song]

On Tue, Jul 28, 2026 at 8:27 PM Zicheng Wang <wangzicheng@honor.com> wrote:
>
> Hi Barry,
>
> > The original code is a bit odd, as it compares a single young
> > generation and a single old generation against the total, rather than
> > comparing all young generations with all old generations.
> >
> > By comparing all young generations against all old generations, we no
> > longer depend on the exact value of MIN_NR_GENS. The idea is to
> > emulate inactive_is_low().
>
> Makes sense, the inactive_is_low() framing fits.
>
> One gap: inactive_is_low()'s ratio is dynamic calculated.
> int_sqrt(10*1024) = 101 (ages only when inactive drops below ~1%),
> while patch 4/5's fixed 4 ages once old falls under 20% of young.
> It seems there will be considerable cost on big memcgs, or should
> the ratio scale with size?
>
> Looking forward to the perf data.

I don't have access to a machine with 1 TB of memory—or even
more than 20 GB—to test this. But I think your comment makes
sense. We shouldn't let a machine with a huge amount of memory
in a single lruvec age too aggressively.
So maybe something like this:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ed35c502923d..a6768b9ec8d6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4968,6 +4968,7 @@ static bool lru_gen_imbalanced(struct lruvec
*lruvec, int type,
 {
        struct lru_gen_folio *lrugen = &lruvec->lrugen;
        unsigned long young = 0, old = 0, seq;
+       unsigned long old_ratio, gb;

        /*
         * reclaim is forced to a single type in those cases, so there is
@@ -4989,7 +4990,16 @@ static bool lru_gen_imbalanced(struct lruvec
*lruvec, int type,
                else
                        old += size;
        }
-       return young > old * 4;
+
+       /*
+        * Copied from the active/inactive LRU heuristic, but uses a higher
+        * old_ratio so fewer folios remain in the old generations, avoiding
+        * overly aggressive aging.
+        */
+       gb = (young + old) >> (30 - PAGE_SHIFT);
+       old_ratio = gb ? int_sqrt(10 * gb) : 1;
+       old <<= 2;
+       return young > old * old_ratio;
 }

Best Regards
Barry

Re: [RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

[Lian Wang]

Hi Barry,

I tested a list-based exhaustion check on top of the v2 series.

Using remaining > 0 still misses the case where remaining reaches zero
exactly when the last folio leaves the oldest generation. Since
scan_folios() still holds the lruvec lock, checking the generation's zone
lists directly avoids both this boundary and the scanned/nr_to_scan
page-versus-folio mismatch.

The change below only replaces the exhaustion check. It keeps the
priority-gated fallback unchanged, and reports the MIN_NR_GENS early
return as exhausted to preserve the current v2 behavior.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6a73be7590cd..59b8725a0650 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4635,7 +4635,8 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
 
 static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 		       struct scan_control *sc, int type, int tier,
-		       struct list_head *list, int *isolatedp)
+		       struct list_head *list, int *isolatedp,
+		       bool *scan_exhausted)
 {
 	int i;
 	int gen;
@@ -4650,8 +4651,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
 	VM_WARN_ON_ONCE(!list_empty(list));
 
-	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
+	if (get_nr_gens(lruvec, type) == MIN_NR_GENS) {
+		*scan_exhausted = true;
 		return 0;
+	}
 
 	gen = lru_gen_from_seq(lrugen->min_seq[type]);
 
@@ -4704,6 +4707,14 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 				scanned, skipped, isolated,
 				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
 
+	*scan_exhausted = true;
+	for (i = 0; i < MAX_NR_ZONES; i++) {
+		if (!list_empty(&lrugen->folios[gen][type][i])) {
+			*scan_exhausted = false;
+			break;
+		}
+	}
+
 	*isolatedp = isolated;
 	return scanned;
 }
@@ -4757,11 +4768,13 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	int type = get_type_to_scan(lruvec, swappiness);
 
 	for_each_evictable_type(i, swappiness) {
+		bool scan_exhausted;
 		int scanned;
 		int tier = get_tier_idx(lruvec, type);
 
 		scanned = scan_folios(nr_to_scan, lruvec, sc,
-				      type, tier, list, isolated);
+				      type, tier, list, isolated,
+				      &scan_exhausted);
 
 		total_scanned += scanned;
 		if (*isolated) {
@@ -4770,15 +4783,12 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			break;
 		}
 		/*
-		 * If scanned >= nr_to_scan or isolated >= MIN_LRU_BATCH,
-		 * avoid falling back to the other type. The preferred
-		 * type is still reclaimable; otherwise, it would have
-		 * already run out of reclaimable generations. Falling
-		 * back too readily can disrupt the positive_ctrl_err()
-		 * bias. Also, only fall back when reclaim is running at
-		 * a high priority.
+		 * The preferred type is still reclaimable unless this scan
+		 * target is exhausted. Preserve the existing priority-gated
+		 * fallback without inferring exhaustion from scanned, which
+		 * counts base pages rather than iterations.
 		 */
-		if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
+		if (scan_exhausted) {
 			if (sc->priority > 2)
 				break;
 			type = !type;

I built and booted this on an arm64 VM with MGLRU enabled. A fresh
swappiness=200 pressure smoke with a 2 GiB file-cache setup and about
7 GiB of touched anonymous memory completed without a
WARN/Oops/BUG/panic.

Would this be worth considering for the next revision? If I misunderstood
the intended semantics, please ignore this.

Thanks,
Lian

Re: [RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

[Barry Song]

On Thu, Jul 30, 2026 at 2:41 PM Lian Wang <lianux.mm@gmail.com> wrote:
>
> Hi Barry,
>
> I tested a list-based exhaustion check on top of the v2 series.
>
> Using remaining > 0 still misses the case where remaining reaches zero
> exactly when the last folio leaves the oldest generation. Since
> scan_folios() still holds the lruvec lock, checking the generation's zone
> lists directly avoids both this boundary and the scanned/nr_to_scan
> page-versus-folio mismatch.
>
> The change below only replaces the exhaustion check. It keeps the
> priority-gated fallback unchanged, and reports the MIN_NR_GENS early
> return as exhausted to preserve the current v2 behavior.
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6a73be7590cd..59b8725a0650 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4635,7 +4635,8 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
>
>  static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>                        struct scan_control *sc, int type, int tier,
> -                      struct list_head *list, int *isolatedp)
> +                      struct list_head *list, int *isolatedp,
> +                      bool *scan_exhausted)
>  {
>         int i;
>         int gen;
> @@ -4650,8 +4651,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>         VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
>         VM_WARN_ON_ONCE(!list_empty(list));
>
> -       if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
> +       if (get_nr_gens(lruvec, type) == MIN_NR_GENS) {
> +               *scan_exhausted = true;
>                 return 0;
> +       }
>
>         gen = lru_gen_from_seq(lrugen->min_seq[type]);
>
> @@ -4704,6 +4707,14 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>                                 scanned, skipped, isolated,
>                                 type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>
> +       *scan_exhausted = true;
> +       for (i = 0; i < MAX_NR_ZONES; i++) {
> +               if (!list_empty(&lrugen->folios[gen][type][i])) {
> +                       *scan_exhausted = false;
> +                       break;
> +               }
> +       }

Thanks very much. Your understanding is correct. We do have the
case where the last folio leaves the list while all the other
scan targets have already been satisfied. My understanding is
that this should be a very low-probability case, so it may not
be worth the added code complexity?

> +
>         *isolatedp = isolated;
>         return scanned;
>  }
> @@ -4757,11 +4768,13 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>         int type = get_type_to_scan(lruvec, swappiness);
>
>         for_each_evictable_type(i, swappiness) {
> +               bool scan_exhausted;
>                 int scanned;
>                 int tier = get_tier_idx(lruvec, type);
>
>                 scanned = scan_folios(nr_to_scan, lruvec, sc,
> -                                     type, tier, list, isolated);
> +                                     type, tier, list, isolated,
> +                                     &scan_exhausted);
>
>                 total_scanned += scanned;
>                 if (*isolated) {
> @@ -4770,15 +4783,12 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>                         break;
>                 }
>                 /*
> -                * If scanned >= nr_to_scan or isolated >= MIN_LRU_BATCH,
> -                * avoid falling back to the other type. The preferred
> -                * type is still reclaimable; otherwise, it would have
> -                * already run out of reclaimable generations. Falling
> -                * back too readily can disrupt the positive_ctrl_err()
> -                * bias. Also, only fall back when reclaim is running at
> -                * a high priority.
> +                * The preferred type is still reclaimable unless this scan
> +                * target is exhausted. Preserve the existing priority-gated
> +                * fallback without inferring exhaustion from scanned, which
> +                * counts base pages rather than iterations.
>                  */
> -               if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH) {
> +               if (scan_exhausted) {
>                         if (sc->priority > 2)
>                                 break;
>                         type = !type;
>
> I built and booted this on an arm64 VM with MGLRU enabled. A fresh
> swappiness=200 pressure smoke with a 2 GiB file-cache setup and about
> 7 GiB of touched anonymous memory completed without a
> WARN/Oops/BUG/panic.

Thanks! I'm going to send out RFC v3. I'd really appreciate any
further testing you can do.

>
> Would this be worth considering for the next revision? If I misunderstood
> the intended semantics, please ignore this.

Thanks! Your understanding of the corner case is 100% correct.
It's just that, given how unlikely it is to occur, I don't
think it would have any noticeable impact, so it may not be
worth the added complexity.

Best Regards
Barry

Re: [RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

[Lian Wang]

Hi Barry,

That makes sense. I agree that this rare corner case does not justify
the added complexity for now, so please feel free to leave it out of v3.

I'll test v3, with a particular focus on the generation-imbalance aging
behavior in 4/5, and report back with the results.

Thanks,
Lian

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[wangzicheng]

On 2026-07-30 06:36 +0800, Barry Song wrote:
> On Tue, Jul 28, 2026 at 8:27 PM Zicheng Wang <wangzicheng@honor.com> wrote:
> >
> > Hi Barry,
> >
> > > The original code is a bit odd, as it compares a single young
> > > generation and a single old generation against the total, rather than
> > > comparing all young generations with all old generations.
> > >
> > > By comparing all young generations against all old generations, we no
> > > longer depend on the exact value of MIN_NR_GENS. The idea is to
> > > emulate inactive_is_low().
> >
> > Makes sense, the inactive_is_low() framing fits.
> >
> > One gap: inactive_is_low()'s ratio is dynamic calculated.
> > int_sqrt(10*1024) = 101 (ages only when inactive drops below ~1%),
> > while patch 4/5's fixed 4 ages once old falls under 20% of young.
> > It seems there will be considerable cost on big memcgs, or should
> > the ratio scale with size?
> >
> > Looking forward to the perf data.
> 
> I don't have access to a machine with 1 TB of memory—or even

Me neither.

> more than 20 GB—to test this. But I think your comment makes
> sense. We shouldn't let a machine with a huge amount of memory
> in a single lruvec age too aggressively.
> So maybe something like this:
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ed35c502923d..a6768b9ec8d6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4968,6 +4968,7 @@ static bool lru_gen_imbalanced(struct lruvec
> *lruvec, int type,
>  {
>         struct lru_gen_folio *lrugen = &lruvec->lrugen;
>         unsigned long young = 0, old = 0, seq;
> +       unsigned long old_ratio, gb;
> 
>         /*
>          * reclaim is forced to a single type in those cases, so there is
> @@ -4989,7 +4990,16 @@ static bool lru_gen_imbalanced(struct lruvec
> *lruvec, int type,
>                 else
>                         old += size;
>         }
> -       return young > old * 4;
> +
> +       /*
> +        * Copied from the active/inactive LRU heuristic, but uses a higher
> +        * old_ratio so fewer folios remain in the old generations, avoiding
> +        * overly aggressive aging.
> +        */
> +       gb = (young + old) >> (30 - PAGE_SHIFT);
> +       old_ratio = gb ? int_sqrt(10 * gb) : 1;
> +       old <<= 2;
> +       return young > old * old_ratio;
>  }
Look good to me. 

Also, this heuristic makes me believe more that MGLRU's `aging' could be
exposed as a standalone feature, allowing users to tune it more flexibly.

> Best Regards
> Barry
> 

Best,
Zicheng

Re: [RFC PATCH v2 4/5] mm: mglru: run aging when pages are severely imbalanced across gens

[Kairui Song]

On Sun, Jul 26, 2026 at 8:21 PM Barry Song (Xiaomi) <baohua@kernel.org> wrote:
>
> From: lyugaofei <lyugaofei@xiaomi.com>
>
> This partially restores the reclaim behavior introduced in Yu
> Zhao's initial MGLRU commit, ac35a4902370 ("mm: multi-gen LRU:
> minimal implementation"):
>         /*
>          * It's also ideal to spread pages out evenly, i.e., 1/(MIN_NR_GENS+1)
>          * of the total number of pages for each generation. A reasonable range
>          * for this average portion is [1/MIN_NR_GENS, 1/(MIN_NR_GENS+2)]. The
>          * aging cares about the upper bound of hot pages, while the eviction
>          * cares about the lower bound of cold pages.
>          */
>         if (young * MIN_NR_GENS > total)
>                 return true;
>         if (old * (MIN_NR_GENS + 2) < total)
>                 return true;
>
> But with a stricter condition: the younger generations must contain
> at least four times as many folios as the older generations. This
> allows aging to keep folios of the preferred type spread across the
> reclaimable generations.
>
> Signed-off-by: lyugaofei <lyugaofei@xiaomi.com>
> Co-developed-by: Barry Song (Xiaomi) <baohua@kernel.org>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/vmscan.c | 37 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d6bd64b4dced..7c13dedb0b1f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4956,9 +4956,40 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>         return scanned;
>  }
>
> +static bool lru_gen_imbalanced(struct lruvec *lruvec, int type,
> +               unsigned long max_seq, unsigned long min_seq,
> +               int swappiness)
> +{
> +       struct lru_gen_folio *lrugen = &lruvec->lrugen;
> +       unsigned long young = 0, old = 0, seq;
> +
> +       /*
> +        * reclaim is forced to a single type in those cases, so there is
> +        * no need to consider the swappiness bias
> +        */
> +       if (swappiness == MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
> +               return false;
> +
> +       for (seq = min_seq; seq <= max_seq; seq++) {
> +               int gen = lru_gen_from_seq(seq);
> +               unsigned long size = 0;
> +               int zone;
> +
> +               for (zone = 0; zone < MAX_NR_ZONES; zone++)
> +                       size += max(READ_ONCE(lrugen->nr_pages[gen][type][zone]), 0L);
> +
> +               if (seq + MIN_NR_GENS > max_seq)
> +                       young += size;
> +               else
> +                       old += size;
> +       }
> +       return young > old * 4;

Hi Barry,

MAX_NR_GEN could be tunable, would it be better to use a macro like
MAX_NR_GEN or MIN_NR_GEN here?

And I know there are experiments to extend MGLRU to run aging
periodically so each gen has a fixed age:

https://lwn.net/Articles/976985/

Not sure if this will conflict if this is a hardcoded aging behavior,
and if we want things like that in the future, these are useful on
servers, I think android might benefit foo.

Re: [RFC PATCH v2 5/5] mm: mglru: run aging if the preferred type has no folios in reclaimable gens

[Baolin Wang]

On 7/26/26 8:21 PM, Barry Song (Xiaomi) wrote:
> Respect the type selected by positive_ctrl_err(). If there are no
> reclaimable gens left for that type, run aging.
> 
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>   mm/vmscan.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7c13dedb0b1f..d63322cdb1bc 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4996,6 +4996,10 @@ static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
>   	if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
>   		return true;
>   
> +	/* run aging if the preferred type is exhausted */
> +	if (min_seq[type] + MIN_NR_GENS > max_seq)
> +		return true;
> +
>   	/* try to avoid aging, do gentle reclaim at the default priority */
>   	if (sc->priority == DEF_PRIORITY)
>   		return false;

Looks reasonable to me.

Additionally, for sc->priority > DEF_PRIORITY, is there the same check 
logic for the scan type?

if (min_seq[type] + MIN_NR_GENS == max_seq)
	return true;

Re: [RFC PATCH v2 5/5] mm: mglru: run aging if the preferred type has no folios in reclaimable gens

[Barry Song]

On Fri, Jul 31, 2026 at 3:27 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 7/26/26 8:21 PM, Barry Song (Xiaomi) wrote:
> > Respect the type selected by positive_ctrl_err(). If there are no
> > reclaimable gens left for that type, run aging.
> >
> > Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> > ---
> >   mm/vmscan.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 7c13dedb0b1f..d63322cdb1bc 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4996,6 +4996,10 @@ static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
> >       if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
> >               return true;
> >
> > +     /* run aging if the preferred type is exhausted */
> > +     if (min_seq[type] + MIN_NR_GENS > max_seq)
> > +             return true;
> > +
> >       /* try to avoid aging, do gentle reclaim at the default priority */
> >       if (sc->priority == DEF_PRIORITY)
> >               return false;
>
> Looks reasonable to me.

Yes, this is reasonable. However, I also noticed that it can
slightly increase system time at moderate swappiness values
during a kernel build.

At swappiness values near 100, overall performance is more
important than strictly preserving the swappiness bias. To
address this, I refined the behavior for balanced swappiness
in v3:
https://lore.kernel.org/linux-mm/20260731083843.37811-7-baohua@kernel.org/

>
> Additionally, for sc->priority > DEF_PRIORITY, is there the same check
> logic for the scan type?
>
> if (min_seq[type] + MIN_NR_GENS == max_seq)
>         return true;

We also have an imbalance check: if there are only three
generations (min_seq[type] + MIN_NR_GENS == max_seq) and the
folio distribution is heavily skewed across them, we trigger
aging. This helps preserve the intended swappiness bias:

https://lore.kernel.org/linux-mm/20260731083843.37811-6-baohua@kernel.org/

Thanks
Barry