Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages_bulk()

[Michal Hocko]

On Tue 11-08-20 18:28:23, Charan Teja Reddy wrote:
[...]
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e4896e6..839039f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1304,6 +1304,11 @@ static void free_pcppages_bulk(struct zone *zone, int 
> count,
>       struct page *page, *tmp;
>       LIST_HEAD(head);
>  
> +     /*
> +      * Ensure proper count is passed which otherwise would stuck in the
> +      * below while (list_empty(list)) loop.
> +      */
> +     count = min(pcp->count, count);
>       while (count) {
>               struct list_head *list;


How does this prevent the race actually? Don't we need something like
the following instead?
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e028b87ce294..45bcc7ba37c4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1317,9 +1317,16 @@ static void free_pcppages_bulk(struct zone *zone, int 
count,
                 * lists
                 */
                do {
+                       bool looped = false;
+
                        batch_free++;
-                       if (++migratetype == MIGRATE_PCPTYPES)
+                       if (++migratetype == MIGRATE_PCPTYPES) {
+                               if (looped)
+                                       goto free;
+
                                migratetype = 0;
+                               looped = true;
+                       }
                        list = &pcp->lists[migratetype];
                } while (list_empty(list));
 
@@ -1352,6 +1359,7 @@ static void free_pcppages_bulk(struct zone *zone, int 
count,
                } while (--count && --batch_free && !list_empty(list));
        }
 
+free:
        spin_lock(&zone->lock);
        isolated_pageblocks = has_isolate_pageblock(zone);
 
-- 
Michal Hocko
SUSE Labs