From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: 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 11:31:07 +0200 [thread overview]
Message-ID: <573C365B.6020807@suse.cz> (raw)
In-Reply-To: <20160518092100.GB2527@techsingularity.net>
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?
>
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WARNING: multiple messages have this Message-ID (diff)
From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@techsingularity.net>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: 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 11:31:07 +0200 [thread overview]
Message-ID: <573C365B.6020807@suse.cz> (raw)
In-Reply-To: <20160518092100.GB2527@techsingularity.net>
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?
>
next prev parent reply other threads:[~2016-05-18 9:31 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 [this message]
2016-05-18 9:31 ` Vlastimil Babka
2016-05-18 9:52 ` Mel Gorman
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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