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 11:31:07 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, 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 05/18/2016 11:21 AM, Mel Gorman wrote:
> On Tue, May 17, 2016 at 04:42:55PM +0900, 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.
>>
>
> I'm not quite getting this one. Minimally, instead of = __PG_HWPOISON, it
> should have been (bad_flags & __PG_POISON). As Vlastimil already pointed
> out, __PG_HWPOISON can be 0. What I'm not getting is why this fixes the
> race. The current race is
>
> 1. Check poison, set bad_flags
> 2. poison clears in parallel
> 3. Check page->flag state in bad_page and trigger warning
>
> The code changes it to
>
> 1. Check poison, set bad_flags
> 2. poison clears in parallel
> 3. Check bad_flags and trigger warning

I think you got step 3 here wrong. It's "skip the warning since we have 
set bad_flags to hwpoison and bad_flags didn't change due to parallel 
unpoison".

Perhaps the question is why do we need to split the handling between 
check_new_page_bad() and bad_page() like this? It might have been 
different in the past, but seems like at this point we only look for 
hwpoison from check_new_page_bad(). But a cleanup can come later.

> There is warning either way. What did I miss?
>