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

Michal Hocko
2020-08-13T11:44:16Z

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