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 10:52:51 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	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 Wed, May 18, 2016 at 11:31:07AM +0200, Vlastimil Babka wrote:
> 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".
> 

I think the benefit is marginal. The race means that the patch will trigger
a warning that might have been missed before due to a parallel unpoison
but that's not necessary a Good Thing. It's inherently race-prone.

Naoya, if you fix the check to (bad_flags & __PG_POISON) then I'll add my
ack but I'm not convinced it's a real problem.

-- 
Mel Gorman
SUSE Labs