Re: [PATCH v9 11/16] mm/mglru: Run aging when one type is drained

["Barry Song (Xiaomi)" <baohua@kernel.org>]

On Sun, Nov 15, 2026 at 4:50 AM <sashiko-bot@kernel.org> wrote:
>
> Thank you for your contribution! Sashiko AI review found 2 potential
> issues to consider:
> - [Medium] mm/mglru: scan_folios() scans seq below min_seq at
>   DEF_PRIORITY

I guess reparenting could be a corner case where it's difficult to
properly preserve the cold/hot ordering.

As mentioned in my earlier reply, I discussed this with Kairui,
and we still prefer having `lruvec_populated_min_seq()` return
`max_seq` instead of `min_seq`:

https://lore.kernel.org/linux-mm/CAGsJ_4xbDy8Qy=5yqmn+59jfxPm3WOgAH7zX9gwhJyO-6fiT6A@mail.gmail.com/

But for sure I can skip the scanning in scan_folios()
if (seq < lrugen->min_seq[type]).


> - [Low] mm/mglru: missed eviction retry in try_to_shrink_lruvec() on
>   concurrent aging
> --

[...]

> When a concurrent reclaimer advances max_seq after scan_folios() finds no
> folios and before new_seq is read, do_drained_age is skipped because
> max_seq != new_seq and should_run_aging() is evaluated against the already
> incremented new_seq.  Comparing lrugen.max_seq against new_seq then sees no
> change and breaks out of try_to_shrink_lruvec() without retrying eviction
> on the newly aged generation.

I actually considered this before sending the patch. I think either
approach is fine, and comparing against `max_seq` instead of `new_seq`
would also be reasonable.

Comparing against `new_seq` still gives us a chance to retry, since
we call `try_to_inc_max_seq()` here, which may advance
`lruvec->lrugen.max_seq` beyond `new_seq`. By the time we reach this
check, `new_seq != READ_ONCE(lruvec->lrugen.max_seq)` may already
be true.

Overall, I think the following fix should address both comments:

From: "Barry Song (Xiaomi)" <baohua@kernel.org>
Date: Sun, 15 Nov 2026 10:24:32 +1300
Subject: [PATCH] Fix-mm-mglru-Run-aging-when-one-type-is-drained.patch

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a6c6c7c1c5e1..a5b8244d4768 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4788,6 +4788,9 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 		seq = lrugen->max_seq;
 
 	seq = lruvec_populated_min_seq(lruvec, type, sc->reclaim_idx + 1, seq);
+	if (seq < lrugen->min_seq[type])
+		return 0;
+
 	*scanned_seq = seq;
 	gen = lru_gen_from_seq(seq);
 	max_gen = lru_gen_from_seq(max_seq);
@@ -5278,7 +5281,7 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 
 		if (!scanned) {
 			/* Retry once if aging advanced max_seq */
-			if (retried || new_seq == READ_ONCE(lruvec->lrugen.max_seq))
+			if (retried || max_seq == READ_ONCE(lruvec->lrugen.max_seq))
 				break;
 			retried = true;
 		}
-- 
2.34.1

>
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20261114200803.23129-1-baohua@kernel.org?part=11