[PATCH v1] mm/vmscan: Add retry logic for cgroups with memory.low in kswapd

[Jiayuan Chen <jiayuan.chen@linux.dev>]

We can set memory.low for cgroups as a soft protection limit. When the
kernel cannot reclaim any pages from other cgroups, it retries reclaim
while ignoring the memory.low protection of the skipped cgroups.

Currently, this retry logic only works in direct reclaim path, but is
missing in the kswapd asynchronous reclaim. Typically, a cgroup may
contain some cold pages that could be reclaimed even when memory.low is
set.

This change adds retry logic to kswapd: if the first reclaim attempt fails
to reclaim any pages and some cgroups were skipped due to memory.low
protection, kswapd will perform a second reclaim pass ignoring memory.low
restrictions.

This ensures more consistent reclaim behavior between direct reclaim and
kswapd. By allowing kswapd to reclaim more proactively from protected
cgroups under global memory pressure, this optimization can help reduce
the occurrence of direct reclaim, which is more disruptive to application
performance.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 mm/vmscan.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c80fcae7f2a1..231c66fcdfd8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -7147,6 +7147,13 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
 		goto restart;
 	}
 
+	/* Restart if we skipped the memory low event */
+	if (sc.memcg_low_skipped && !sc.memcg_low_reclaim &&
+	    sc.priority < 1) {
+		sc.memcg_low_reclaim = 1;
+		goto restart;
+	}
+
 	if (!sc.nr_reclaimed)
 		atomic_inc(&pgdat->kswapd_failures);
 
-- 
2.43.0