linux-kernel - [PATCH linux-next v2] mm: workingset: update description of the source file

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Date:   Thu, 13 Apr 2023 16:34:49 +0800 (CST)
From:   <yang.yang29@....com.cn>
To:     <akpm@...ux-foundation.org>, <hannes@...xchg.org>
Cc:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <iamjoonsoo.kim@....com>
Subject: [PATCH linux-next v2] mm: workingset: update description of the source file

From: Yang Yang <yang.yang29@....com.cn>

The calculation of workingset size is the core logic of handling refault,
it had been updated several times[1][2] after workingset.c was created[3].
But the description hadn't been updated accordingly, this mismatch may
confuse the readers.
So we update the description to make it consistent to the code. 

[1] commit 34e58cac6d8f ("mm: workingset: let cache workingset challenge anon")
[2] commit aae466b0052e ("mm/swap: implement workingset detection for anonymous LRU")
[3] commit a528910e12ec ("mm: thrash detection-based file cache sizing")

Signed-off-by: Yang Yang <yang.yang29@....com.cn>
---
change for v2 - Update commit of workingset_refault() suggested Johannes Weiner.
See https://lore.kernel.org/all/20230316143007.GC116016@cmpxchg.org/
---
 mm/workingset.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index a304e8571d54..b864eec49ddd 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -111,9 +111,20 @@
  *
  *   NR_inactive + (R - E) <= NR_inactive + NR_active
  *
- * which can be further simplified to
+ * If we have swap we should consider about NR_inactive_anon and
+ * NR_active_anon, so for page cache and anonymous respectively:
  *
- *   (R - E) <= NR_active
+ *   NR_inactive_file + (R - E) <= NR_inactive_file + NR_active_file
+ *   + NR_inactive_anon + NR_active_anon
+ *
+ *   NR_inactive_anon + (R - E) <= NR_inactive_anon + NR_active_anon
+ *   + NR_inactive_file + NR_active_file
+ *
+ * Which can be further simplified to:
+ *
+ *   (R - E) <= NR_active_file + NR_inactive_anon + NR_active_anon
+ *
+ *   (R - E) <= NR_active_anon + NR_inactive_file + NR_active_file
  *
  * Put into words, the refault distance (out-of-cache) can be seen as
  * a deficit in inactive list space (in-cache).  If the inactive list
@@ -130,14 +141,14 @@
  * are no longer in active use.
  *
  * So when a refault distance of (R - E) is observed and there are at
- * least (R - E) active pages, the refaulting page is activated
- * optimistically in the hope that (R - E) active pages are actually
+ * least (R - E) pages in the userspace workingset, the refaulting page
+ * is activated optimistically in the hope that (R - E) pages are actually
  * used less frequently than the refaulting page - or even not used at
  * all anymore.
  *
  * That means if inactive cache is refaulting with a suitable refault
  * distance, we assume the cache workingset is transitioning and put
- * pressure on the current active list.
+ * pressure on the current workingset.
  *
  * If this is wrong and demotion kicks in, the pages which are truly
  * used more frequently will be reactivated while the less frequently
@@ -468,7 +479,7 @@ void workingset_refault(struct folio *folio, void *shadow)
 	 * don't activate pages that couldn't stay resident even if
 	 * all the memory was available to the workingset. For page
 	 * cache whether workingset competition needs to consider
-	 * anon or not depends on having swap.
+	 * anon or not depends on having free swap sapce.
 	 */
 	workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
 	/* For anonymous page */
-- 
2.25.1