Re: [PATCH v7 00/15] mm/mglru: improve reclaim loop and dirty folio handling

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

On Tue, 28 Apr 2026 02:06:51 +0800 Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> wrote:

> From: Kairui Song <kasong@tencent.com>
> 
> This series cleans up and slightly improves MGLRU's reclaim loop and
> dirty writeback handling. As a result, we can see an up to ~30% increase
> in some workloads like MongoDB with YCSB and a huge decrease in file
> refault, no swap involved. Other common benchmarks have no regression,
> and LOC is reduced, with less unexpected OOM, too.

Thanks, I've updated mm.git's mm-new branch to this version.

> Changes in v7:
> - Fix swappiness not being effective with a standalone fix patch
>   from Barry Song. It's OK to be a standalone fix since that is not a
>   major bug but an unexpected behavior change, and shouldn't effect any
>   bisecting. I slightly adjusted the commit message as the subjcect is too
>   long and getting truncated for mail:
>   https://lore.kernel.org/linux-mm/20260425205759.1701-1-baohua@kernel.org/
> - Remove the min limit for calculating nr_to_scan:
>   https://lore.kernel.org/linux-mm/aet1hd9DfRH4aSOO@KASONG-MC4/
>   Instead just revert to V1:
>   https://sashiko.dev/#/message/20260318-mglru-reclaim-v1-3-2c46f9eb0508%40tencent.com
>   Everyone was fine with that, the min limit in later version was
>   introduced to cover sashiko's review on V1, but now think again, that's
>   actually not a bug and instead could be beneficial. This min
>   check doesn't always make sense and there isn't any practical issue observed.
> - Retest still looking very good in every case.

Here's how v7 altered mm.git.   (Looks small - did I mess this up?)


--- a/mm/vmscan.c~b
+++ a/mm/vmscan.c
@@ -4788,8 +4788,13 @@ static int isolate_folios(unsigned long
 			*isolate_scanned = scanned;
 			break;
 		}
-
-		type = !type;
+		/*
+		 * 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)
+			type = !type;
 	}
 
 	return total_scanned;
@@ -4909,18 +4914,14 @@ static long get_nr_to_scan(struct lruvec
 	unsigned long nr_to_scan, evictable;
 
 	evictable = lruvec_evictable_size(lruvec, swappiness);
-	nr_to_scan = evictable;
 
 	/* try to scrape all its memory if this memcg was deleted */
 	if (!mem_cgroup_online(memcg))
-		return nr_to_scan;
+		return evictable;
 
-	nr_to_scan = apply_proportional_protection(memcg, sc, nr_to_scan);
+	nr_to_scan = apply_proportional_protection(memcg, sc, evictable);
 	nr_to_scan >>= sc->priority;
 
-	if (!nr_to_scan && sc->priority < DEF_PRIORITY)
-		nr_to_scan = min(evictable, SWAP_CLUSTER_MAX);
-
 	return nr_to_scan;
 }
 
_