[PATCH] mm: Add more comments for MADV_FREE

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

To help people understand the MADV_FREE code, especially the page flag,
PG_swapbacked.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@surriel.com>
---
 include/linux/mm_inline.h  | 9 +++++----
 include/linux/page-flags.h | 4 ++++
 mm/swap.c                  | 6 +++---
 3 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 6f2fef7b0784..01144dd02a5f 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -9,10 +9,11 @@
  * page_is_file_cache - should the page be on a file LRU or anon LRU?
  * @page: the page to test
  *
- * Returns 1 if @page is page cache page backed by a regular filesystem,
- * or 0 if @page is anonymous, tmpfs or otherwise ram or swap backed.
- * Used by functions that manipulate the LRU lists, to sort a page
- * onto the right LRU list.
+ * Returns 1 if @page is page cache page backed by a regular filesystem or
+ * anonymous page lazily freed (e.g. via MADV_FREE).  Returns 0 if @page is
+ * normal anonymous page, tmpfs or otherwise ram or swap backed.  Used by
+ * functions that manipulate the LRU lists, to sort a page onto the right LRU
+ * list.
  *
  * We would like to get this info without a page flag, but the state
  * needs to survive until the page is last deleted from the LRU, which
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 49c2697046b9..992b71c4fc85 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -63,6 +63,10 @@
  * page_waitqueue(page) is a wait queue of all tasks waiting for the page
  * to become unlocked.
  *
+ * PG_swapbacked is cleared if the page is page cache page backed by a regular
+ * file system or anonymous page lazily freed (e.g. via MADV_FREE).  It is set
+ * if the page is normal anonymous page, tmpfs or otherwise RAM or swap backed.
+ *
  * PG_uptodate tells whether the page's contents is valid.  When a read
  * completes, the page becomes uptodate, unless a disk I/O error happened.
  *
diff --git a/mm/swap.c b/mm/swap.c
index 6a8be910b14d..90bb14695809 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -573,9 +573,9 @@ static void lru_lazyfree_fn(struct page *page, struct lruvec *lruvec,
 		ClearPageActive(page);
 		ClearPageReferenced(page);
 		/*
-		 * lazyfree pages are clean anonymous pages. They have
-		 * SwapBacked flag cleared to distinguish normal anonymous
-		 * pages
+		 * Lazyfree pages are clean anonymous pages.  They
+		 * have PG_swapbacked flag cleared, to distinguish
+		 * them from normal anonymous pages
 		 */
 		ClearPageSwapBacked(page);
 		add_page_to_lru_list(page, lruvec, LRU_INACTIVE_FILE);
-- 
2.25.0