[PATCH] mm: workingset: clarify eviction order and distance calculation

From: Oscar Salvador <osalvador@suse.de>
To: akpm@linux-foundation.org
Cc: hannes@cmpxchg.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Oscar Salvador <osalvador@suse.de>
Subject: [PATCH] mm: workingset: clarify eviction order and distance calculation
Date: Mon,  1 Feb 2021 07:06:51 +0100	[thread overview]
Message-ID: <20210201060651.3781-1-osalvador@suse.de> (raw)

The premise of the refault distance is that it can be seen as a deficit
of the inactive list space, so that if the inactive list would have had
(R - E) more slots, the page would not have been evicted but promoted
to the active list instead.

However, the way the code is ordered right now set us to be off by one,
so the real number of slots would be (R - E) + 1.
I stumbled upon this when trying to understand the code and it puzzled me
that the comments did not match what the code did.

This it not an issue at all since evictions and refaults tend to happen
in a number large enough that being off-by-one does not have any impact
- and since the compiler and CPUs are free to rearrange the execution
sequence anyway.
But as Johannes says, it is better to re-arrange the code in the proper
order since otherwise would be misleading to somebody who is actively
reading and trying to understand the logic of the code - like it
happened to me.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/workingset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 10e96de945b3..0201aa1ff320 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -263,10 +263,10 @@ void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 
 	lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
-	workingset_age_nonresident(lruvec, thp_nr_pages(page));
 	/* XXX: target_memcg can be NULL, go through lruvec */
 	memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
 	eviction = atomic_long_read(&lruvec->nonresident_age);
+	workingset_age_nonresident(lruvec, thp_nr_pages(page));
 	return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page));
 }
 
-- 
2.26.2

