mm: workingset: update description of the source file

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 34e58cac ("mm: workingset: let cache workingset challenge anon")
[2] commit aae466b0 ("mm/swap: implement workingset detection for anonymous LRU")
[3] commit a528910e ("mm: thrash detection-based file cache sizing")

Link: https://lkml.kernel.org/r/202304131634494948454@zte.com.cnSigned-off-by: Yang Yang <yang.yang29@zte.com.cn>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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
@@ -469,7 +480,7 @@ 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. Whether
 	 * workingset competition needs to consider anon or not depends
-	 * on having swap.
+	 * on having free swap space.
 	 */
 	workingset_size = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
 	if (!file) {