linux-kernel - [PATCH V1] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

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Message-Id: <20250313034812.3910627-1-hezhongkun.hzk@bytedance.com>
Date: Thu, 13 Mar 2025 11:48:12 +0800
From: Zhongkun He <hezhongkun.hzk@...edance.com>
To: akpm@...ux-foundation.org
Cc: mhocko@...e.com,
	hannes@...xchg.org,
	muchun.song@...ux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Zhongkun He <hezhongkun.hzk@...edance.com>
Subject: [PATCH V1] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

With this patch 'commit <68cd9050d871> ("mm: add swappiness= arg to
memory.reclaim")', we can submit an additional swappiness=<val> argument
to memory.reclaim. It is very useful because we can dynamically adjust
the reclamation ratio based on the anonymous folios and file folios of
each cgroup. For example,when swappiness is set to 0, we only reclaim
from file folios.

However,we have also encountered a new issue: when swappiness is set to
the MAX_SWAPPINESS, it may still only reclaim file folios. This is due
to the knob of cache_trim_mode, which depends solely on the ratio of
inactive folios, regardless of whether there are a large number of cold
folios in anonymous folio list.

So, we hope to add a new control logic where proactive memory reclaim only
reclaims from anonymous folios when swappiness is set to MAX_SWAPPINESS.
For example, something like this:

echo "2M swappiness=200" > /sys/fs/cgroup/memory.reclaim

will perform reclaim on the rootcg with a swappiness setting of 200 (max
swappiness) regardless of the file folios. Users have a more comprehensive
view of the application's memory distribution because there are many
metrics available. For example, if we find that a certain cgroup has a
large number of inactive anon folios, we can reclaim only those and skip
file folios, because with the zram/zswap, the IO tradeoff that
cache_trim_mode is making doesn't hold - file refaults will cause IO,
whereas anon decompression will not.

With this patch, the swappiness argument of memory.reclaim has a more
precise semantics: 0 means reclaiming only from file pages, while 200
means reclaiming just from anonymous pages.

V1:
  Update Documentation/admin-guide/cgroup-v2.rst --from Andrew Morton
  Add more descriptions in the comment.   --from Johannes Weiner

Signed-off-by: Zhongkun He <hezhongkun.hzk@...edance.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
---
 Documentation/admin-guide/cgroup-v2.rst |  4 ++++
 mm/vmscan.c                             | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index cb1b4e759b7e..6a4487ead7e0 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1343,6 +1343,10 @@ The following nested keys are defined.
 	same semantics as vm.swappiness applied to memcg reclaim with
 	all the existing limitations and potential future extensions.

+	The swappiness have the range [0, 200], 0 means reclaiming only
+	from file folios, 200 (MAX_SWAPPINESS) means reclaiming just from
+	anonymous folios.
+
   memory.peak
 	A read-write single value file which exists on non-root cgroups.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c767d71c43d7..f4312b41e0e0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2438,6 +2438,16 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 		goto out;
 	}

+	/*
+	 * Do not bother scanning file folios if the memory reclaim
+	 * invoked by userspace through memory.reclaim and the
+	 * swappiness is MAX_SWAPPINESS.
+	 */
+	if (sc->proactive && (swappiness == MAX_SWAPPINESS)) {
+		scan_balance = SCAN_ANON;
+		goto out;
+	}
+
 	/*
 	 * Do not apply any pressure balancing cleverness when the
 	 * system is close to OOM, scan both anon and file equally
-- 
2.39.5