linux-kernel - Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page

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Date:	Wed, 18 May 2016 09:30:41 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags
 for hwpoison page

On 05/17/2016 09:42 AM, Naoya Horiguchi wrote:
> There's a race window between checking page->flags and unpoisoning, which
> taints kernel with "BUG: Bad page state". That's overkill. It's safer to
> use bad_flags to detect hwpoisoned page.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> ---
>   mm/page_alloc.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git tmp/mm/page_alloc.c tmp_patched/mm/page_alloc.c
> index 5b269bc..4e0fa37 100644
> --- tmp/mm/page_alloc.c
> +++ tmp_patched/mm/page_alloc.c
> @@ -522,8 +522,8 @@ static void bad_page(struct page *page, const char *reason,
>   	static unsigned long nr_shown;
>   	static unsigned long nr_unshown;
>
> -	/* Don't complain about poisoned pages */
> -	if (PageHWPoison(page)) {
> +	/* Don't complain about hwpoisoned pages */
> +	if (bad_flags == __PG_HWPOISON) {

This will wrongly return prematurely on !CONFIG_MEMORY_FAILURE where 
__PG_HWPOISON == 0 and bad_page() called for other reasons than bad flags?

>   		page_mapcount_reset(page); /* remove PageBuddy */
>   		return;
>   	}
>