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

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Message-ID: <20160518092100.GB2527@techsingularity.net>
Date:	Wed, 18 May 2016 10:21:01 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, 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 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

There is warning either way. What did I miss?

-- 
Mel Gorman
SUSE Labs