linux-kernel - Re: [PATCH] mm, page_alloc: prevent infinite loop in buffered

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Message-ID: <a4da34ff-cda2-a9a9-d586-277eb6f8797e@suse.cz>
Date:	Mon, 30 May 2016 11:46:05 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, page_alloc: prevent infinite loop in
 buffered_rmqueue()

On 05/30/2016 11:01 AM, Mel Gorman wrote:
> From: Vlastimil Babka <vbabka@...e.cz>
>
> 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

That was a wrong one, which I corrected later. Also it's no longer 
mmotm. Correction below:

Fixes: 479f854a207c ("mm, page_alloc: defer debugging checks of pages 
allocated from the PCP")

> Reported-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Thanks Mel, I've missed that the patch didn't go in.

> ---
>  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 f8f3bfc435ee..bb320cde4d6d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2609,11 +2609,12 @@ 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
>