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

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  [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-17 09:50 +0200
    Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags  for hwpoison page Vlastimil Babka <vbabka@suse.cz> - 2016-05-18 09:40 +0200
    Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 11:30 +0200
      Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags  for hwpoison page Vlastimil Babka <vbabka@suse.cz> - 2016-05-18 11:40 +0200
        Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 12:00 +0200
          Re: [PATCH v1] mm: bad_page() checks bad_flags instead of  page->flags for hwpoison page Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-18 12:20 +0200
        [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-05-18 12:20 +0200
          Re: [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Mel Gorman <mgorman@techsingularity.net> - 2016-05-18 16:10 +0200
            Re: [PATCH v2] mm: check_new_page_bad() directly returns in  __PG_HWPOISON case Vlastimil Babka <vbabka@suse.cz> - 2016-05-20 16:30 +0200

#1402166 — [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page

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.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 mm/page_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git tmp/mm/page_alloc.c tmp_patched/mm/page_alloc.c
index 5b269bc..4e0fa37 100644
--- tmp/mm/page_alloc.c
+++ tmp_patched/mm/page_alloc.c
@@ -522,8 +522,8 @@ static void bad_page(struct page *page, const char *reason,
 	static unsigned long nr_shown;
 	static unsigned long nr_unshown;
 
-	/* Don't complain about poisoned pages */
-	if (PageHWPoison(page)) {
+	/* Don't complain about hwpoisoned pages */
+	if (bad_flags == __PG_HWPOISON) {
 		page_mapcount_reset(page); /* remove PageBuddy */
 		return;
 	}
-- 
2.7.0

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#1402763 — Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page

On 05/17/2016 09:42 AM, 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.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>   mm/page_alloc.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git tmp/mm/page_alloc.c tmp_patched/mm/page_alloc.c
> index 5b269bc..4e0fa37 100644
> --- tmp/mm/page_alloc.c
> +++ tmp_patched/mm/page_alloc.c
> @@ -522,8 +522,8 @@ static void bad_page(struct page *page, const char *reason,
>   	static unsigned long nr_shown;
>   	static unsigned long nr_unshown;
>
> -	/* Don't complain about poisoned pages */
> -	if (PageHWPoison(page)) {
> +	/* Don't complain about hwpoisoned pages */
> +	if (bad_flags == __PG_HWPOISON) {

This will wrongly return prematurely on !CONFIG_MEMORY_FAILURE where 
__PG_HWPOISON == 0 and bad_page() called for other reasons than bad flags?

>   		page_mapcount_reset(page); /* remove PageBuddy */
>   		return;
>   	}
>

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#1402833 — 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

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#1402836 — 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?
>

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#1402844 — 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

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#1402859 — Re: [PATCH v1] mm: bad_page() checks bad_flags instead of page->flags for hwpoison page

On Wed, May 18, 2016 at 10:52:51AM +0100, Mel Gorman wrote:
> 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.

This v1 had the wrong operator issue as you mentioned. I posted v2 a while ago,
which has no such issue and is a better fix hopefully.

Thanks,
Naoya Horiguchi

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#1402861 — [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON case

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".
> 
> 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.

Thanks for clarification. check_new_page_bad() is the only function interested
in hwpoison flag, so we had better move the hwpoison related code in bad_page()
to check_new_page_bad().

Thanks,
Naoya Horiguchi
---
From c600b1ee6c36b3df6973f5365b4179c92f3c08e3 Mon Sep 17 00:00:00 2001
From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Date: Wed, 18 May 2016 18:42:57 +0900
Subject: [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON
 case

Currently we check page->flags twice for "HWPoisoned" case of
check_new_page_bad(), which can cause a race with unpoisoning.
This race unnecessarily taints kernel with "BUG: Bad page state".
check_new_page_bad() is the only caller of bad_page() which is interested
in __PG_HWPOISON, so let's move the hwpoison related code in bad_page()
to it.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 mm/page_alloc.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5b269bc3eca7..59b938ddfb2d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -522,12 +522,6 @@ static void bad_page(struct page *page, const char *reason,
 	static unsigned long nr_shown;
 	static unsigned long nr_unshown;
 
-	/* Don't complain about poisoned pages */
-	if (PageHWPoison(page)) {
-		page_mapcount_reset(page); /* remove PageBuddy */
-		return;
-	}
-
 	/*
 	 * Allow a burst of 60 reports, then keep quiet for that minute;
 	 * or allow a steady drip of one report per second.
@@ -1654,6 +1648,9 @@ static void check_new_page_bad(struct page *page)
 	if (unlikely(page->flags & __PG_HWPOISON)) {
 		bad_reason = "HWPoisoned (hardware-corrupted)";
 		bad_flags = __PG_HWPOISON;
+		/* Don't complain about hwpoisoned pages */
+		page_mapcount_reset(page); /* remove PageBuddy */
+		return;
 	}
 	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
 		bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
-- 
2.5.5

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#1403010 — Re: [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON case

On Wed, May 18, 2016 at 10:09:50AM +0000, Naoya Horiguchi wrote:
> From c600b1ee6c36b3df6973f5365b4179c92f3c08e3 Mon Sep 17 00:00:00 2001
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Wed, 18 May 2016 18:42:57 +0900
> Subject: [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON
>  case
> 
> Currently we check page->flags twice for "HWPoisoned" case of
> check_new_page_bad(), which can cause a race with unpoisoning.
> This race unnecessarily taints kernel with "BUG: Bad page state".
> check_new_page_bad() is the only caller of bad_page() which is interested
> in __PG_HWPOISON, so let's move the hwpoison related code in bad_page()
> to it.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

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#1404507 — Re: [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON case

On 05/18/2016 04:03 PM, Mel Gorman wrote:
> On Wed, May 18, 2016 at 10:09:50AM +0000, Naoya Horiguchi wrote:
>>  From c600b1ee6c36b3df6973f5365b4179c92f3c08e3 Mon Sep 17 00:00:00 2001
>> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Date: Wed, 18 May 2016 18:42:57 +0900
>> Subject: [PATCH v2] mm: check_new_page_bad() directly returns in __PG_HWPOISON
>>   case
>>
>> Currently we check page->flags twice for "HWPoisoned" case of
>> check_new_page_bad(), which can cause a race with unpoisoning.
>> This race unnecessarily taints kernel with "BUG: Bad page state".
>> check_new_page_bad() is the only caller of bad_page() which is interested
>> in __PG_HWPOISON, so let's move the hwpoison related code in bad_page()
>> to it.
>>
>> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
>
Acked-by: Vlastimil Babka <vbabka@suse.cz>

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