mm: memcontrol: clarify the uncharge_list() loop

uncharge_list() does an unusual list walk because the function can take
regular lists with dedicated list_heads as well as singleton lists where
a single page is passed via the page->lru list node.

This can sometimes lead to confusion as well as suggestions to replace
the loop with a list_for_each_entry(), which wouldn't work.

Signed-off-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 4 insertions, 0 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8614e0d750e5..fa7bf354ae32 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5420,6 +5420,10 @@ static void uncharge_list(struct list_head *page_list)
 	struct list_head *next;
 	struct page *page;
 
+	/*
+	 * Note that the list can be a single page->lru; hence the
+	 * do-while loop instead of a simple list_for_each_entry().
+	 */
 	next = page_list->next;
 	do {
 		unsigned int nr_pages = 1;