[RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset_refault()

[Yosry Ahmed]

In workingset_refault(), we call mem_cgroup_flush_stats_delayed() to
flush stats within an RCU read section and with sleeping disallowed.
Move the call to mem_cgroup_flush_stats_delayed() above the RCU read
section and allow sleeping to avoid unnecessarily performing a lot of
work without sleeping.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---

A lot of code paths call into workingset_refault(), so I am not
generally sure at all whether it's okay to sleep in all contexts or not.
Feedback here would be very helpful.

---
 mm/workingset.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 042eabbb43f6..410bc6684ea7 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
 	unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
 	eviction <<= bucket_order;
 
+	/* Flush stats (and potentially sleep) before holding RCU read lock */
+	mem_cgroup_flush_stats_delayed(true);
 	rcu_read_lock();
 	/*
 	 * Look up the memcg associated with the stored ID. It might
@@ -461,9 +463,6 @@ void workingset_refault(struct folio *folio, void *shadow)
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
 	mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
-
-	/* Do not sleep with RCU lock held */
-	mem_cgroup_flush_stats_delayed(false);
 	/*
 	 * Compare the distance to the existing workingset size. We
 	 * don't activate pages that couldn't stay resident even if
-- 
2.40.0.rc1.284.g88254d51c5-goog