From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Naoya Horiguchi <nao.horiguchi@gmail.com>
Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page
Date: Wed, 18 May 2016 10:52:51 +0100 [thread overview]
Message-ID: <20160518095251.GD2527@techsingularity.net> (raw)
In-Reply-To: <573C365B.6020807@suse.cz>
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
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WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Naoya Horiguchi <nao.horiguchi@gmail.com>
Subject: Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page
Date: Wed, 18 May 2016 10:52:51 +0100 [thread overview]
Message-ID: <20160518095251.GD2527@techsingularity.net> (raw)
In-Reply-To: <573C365B.6020807@suse.cz>
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
next prev parent reply other threads:[~2016-05-18 9:52 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-17 7:42 [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page Naoya Horiguchi
2016-05-17 7:42 ` Naoya Horiguchi
2016-05-18 7:30 ` Vlastimil Babka
2016-05-18 7:30 ` Vlastimil Babka
2016-05-18 9:21 ` Mel Gorman
2016-05-18 9:21 ` Mel Gorman
2016-05-18 9:31 ` Vlastimil Babka
2016-05-18 9:31 ` Vlastimil Babka
2016-05-18 9:52 ` Mel Gorman [this message]
2016-05-18 9:52 ` Mel Gorman
2016-05-18 10:17 ` Naoya Horiguchi
2016-05-18 10:17 ` Naoya Horiguchi
2016-05-18 10:09 ` [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON case Naoya Horiguchi
2016-05-18 10:09 ` Naoya Horiguchi
2016-05-18 14:03 ` Mel Gorman
2016-05-18 14:03 ` Mel Gorman
2016-05-20 14:27 ` Vlastimil Babka
2016-05-20 14:27 ` Vlastimil Babka
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