linux-kernel - Re: [PATCH 28/28] mm, page_alloc: Defer debugging checks of pages allocated from the PCP

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Date:	Wed, 18 May 2016 09:55:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 28/28] mm, page_alloc: Defer debugging checks of pages
 allocated from the PCP

On 05/18/2016 09:51 AM, Vlastimil Babka wrote:
> ----8<----
>  From f52f5e2a7dd65f2814183d8fd254ace43120b828 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@...e.cz>
> Date: Wed, 18 May 2016 09:41:01 +0200
> Subject: [PATCH] mm, page_alloc: prevent infinite loop in buffered_rmqueue()
> 
> In DEBUG_VM kernel, we can hit infinite loop for order == 0 in
> buffered_rmqueue() when check_new_pcp() returns 1, because the bad page is
> never removed from the pcp list. Fix this by removing the page before retrying.
> Also we don't need to check if page is non-NULL, because we simply grab it from
> the list which was just tested for being non-empty.
> 
> Fixes: http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-page_alloc-defer-debugging-checks-of-freed-pages-until-a-pcp-drain.patch

Wrong.
Fixes: http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-page_alloc-defer-debugging-checks-of-pages-allocated-from-the-pcp.patch

> Reported-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>   mm/page_alloc.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8c81e2e7b172..d5b93e5dd697 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2641,11 +2641,12 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
>   				page = list_last_entry(list, struct page, lru);
>   			else
>   				page = list_first_entry(list, struct page, lru);
> -		} while (page && check_new_pcp(page));
>   
> -		__dec_zone_state(zone, NR_ALLOC_BATCH);
> -		list_del(&page->lru);
> -		pcp->count--;
> +			__dec_zone_state(zone, NR_ALLOC_BATCH);
> +			list_del(&page->lru);
> +			pcp->count--;
> +
> +		} while (check_new_pcp(page));
>   	} else {
>   		/*
>   		 * We most definitely don't want callers attempting to
>