linux-kernel - Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages

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Date:   Thu, 13 Aug 2020 13:41:05 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Charan Teja Reddy <charante@...eaurora.org>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, david@...hat.com,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, vinmenon@...eaurora.org
Subject: Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages_bulk()

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