mm: document is_page_cache_freeable()

Enlighten the reader of this code about what reference count makes a page
cache page freeable.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f5b5f02..5be8107 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -286,6 +286,11 @@ static inline int page_mapping_inuse(struct page *page)
 
 static inline int is_page_cache_freeable(struct page *page)
 {
+       /*
+        * A freeable page cache page is referenced only by the caller
+        * that isolated the page, the page cache radix tree and
+        * optional buffer heads at page->private.
+        */
        return page_count(page) - page_has_private(page) == 2;
 }