* Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?
2011-12-05 16:14 ` Michal Hocko
@ 2011-12-05 17:06 ` Mel Gorman
2011-12-08 2:38 ` Wang Sheng-Hui
2011-12-08 16:24 ` KOSAKI Motohiro
2 siblings, 0 replies; 6+ messages in thread
From: Mel Gorman @ 2011-12-05 17:06 UTC (permalink / raw)
To: Michal Hocko; +Cc: Wang Sheng-Hui, linux-mm, linux-kernel, Andrew Morton
On Mon, Dec 05, 2011 at 05:14:43PM +0100, Michal Hocko wrote:
> From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Mon, 5 Dec 2011 17:04:23 +0100
> Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok
>
> 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when
> memory is low] changed the form how free_pages is calculated but it
> forgot that we used to do free_pages - ((1 << order) - 1) so we ended up
> with off-by-two when calculating free_pages.
>
> Spotted-by: Wang Sheng-Hui <shhuiw@gmail.com>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
--
Mel Gorman
SUSE Labs
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?
2011-12-05 16:14 ` Michal Hocko
2011-12-05 17:06 ` Mel Gorman
@ 2011-12-08 2:38 ` Wang Sheng-Hui
2011-12-08 8:05 ` Michal Hocko
2011-12-08 16:24 ` KOSAKI Motohiro
2 siblings, 1 reply; 6+ messages in thread
From: Wang Sheng-Hui @ 2011-12-08 2:38 UTC (permalink / raw)
To: Michal Hocko; +Cc: linux-mm, Mel Gorman, Andrew Morton
[-- Attachment #1: Type: text/plain, Size: 1789 bytes --]
Sorry, Michal.
2011/12/6 Michal Hocko <mhocko@suse.cz>
> On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote:
> > In line 1459, we have "free_pages -= (1 << order) + 1;".
> > Suppose allocating one 0-order page, here we'll get
> > free_pages -= 1 + 1
> > I wonder why there is a "+ 1"?
>
> Good spot. Check the patch bellow.
> ---
> From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Mon, 5 Dec 2011 17:04:23 +0100
> Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok
>
> 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when
> memory is low] changed the form how free_pages is calculated but it
> forgot that we used to do free_pages - ((1 << order) - 1) so we ended up
> with off-by-two when calculating free_pages.
>
> Spotted-by: Wang Sheng-Hui <shhuiw@gmail.com>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
> mm/page_alloc.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9dd443d..8a2f1b6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int
> order, unsigned long mark,
> long min = mark;
> int o;
>
> - free_pages -= (1 << order) + 1;
> + free_pages -= (1 << order) - 1;
>
I don't understand why there is additional "-1".
Use 0-order allocation as example:
0-order page ---- one 4K page
free_pages should subtract 1. Here, free_pages will subtract 0?
> if (alloc_flags & ALLOC_HIGH)
> min -= min / 2;
> if (alloc_flags & ALLOC_HARDER)
> --
> 1.7.7.3
>
> --
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9
> Czech Republic
>
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?
2011-12-08 2:38 ` Wang Sheng-Hui
@ 2011-12-08 8:05 ` Michal Hocko
0 siblings, 0 replies; 6+ messages in thread
From: Michal Hocko @ 2011-12-08 8:05 UTC (permalink / raw)
To: Wang Sheng-Hui; +Cc: linux-mm, Mel Gorman, Andrew Morton
On Thu 08-12-11 10:38:39, Wang Sheng-Hui wrote:
> Sorry, Michal.
>
> 2011/12/6 Michal Hocko <mhocko@suse.cz>
>
> > On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote:
> > > In line 1459, we have "free_pages -= (1 << order) + 1;".
> > > Suppose allocating one 0-order page, here we'll get
> > > free_pages -= 1 + 1
> > > I wonder why there is a "+ 1"?
> >
> > Good spot. Check the patch bellow.
> > ---
> > From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@suse.cz>
> > Date: Mon, 5 Dec 2011 17:04:23 +0100
> > Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok
> >
> > 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when
> > memory is low] changed the form how free_pages is calculated but it
> > forgot that we used to do free_pages - ((1 << order) - 1) so we ended up
> > with off-by-two when calculating free_pages.
> >
> > Spotted-by: Wang Sheng-Hui <shhuiw@gmail.com>
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
> > ---
> > mm/page_alloc.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 9dd443d..8a2f1b6 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int
> > order, unsigned long mark,
> > long min = mark;
> > int o;
> >
> > - free_pages -= (1 << order) + 1;
> > + free_pages -= (1 << order) - 1;
> >
>
> I don't understand why there is additional "-1".
> Use 0-order allocation as example:
> 0-order page ---- one 4K page
> free_pages should subtract 1. Here, free_pages will subtract 0?
Check out all the conditions for free_pages...
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?
2011-12-05 16:14 ` Michal Hocko
2011-12-05 17:06 ` Mel Gorman
2011-12-08 2:38 ` Wang Sheng-Hui
@ 2011-12-08 16:24 ` KOSAKI Motohiro
2 siblings, 0 replies; 6+ messages in thread
From: KOSAKI Motohiro @ 2011-12-08 16:24 UTC (permalink / raw)
To: Michal Hocko
Cc: Wang Sheng-Hui, linux-mm, linux-kernel, Mel Gorman, Andrew Morton
(12/5/11 11:14 AM), Michal Hocko wrote:
> On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote:
>> In line 1459, we have "free_pages -= (1<< order) + 1;".
>> Suppose allocating one 0-order page, here we'll get
>> free_pages -= 1 + 1
>> I wonder why there is a "+ 1"?
>
> Good spot. Check the patch bellow.
> ---
> From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko<mhocko@suse.cz>
> Date: Mon, 5 Dec 2011 17:04:23 +0100
> Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok
>
> 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when
> memory is low] changed the form how free_pages is calculated but it
> forgot that we used to do free_pages - ((1<< order) - 1) so we ended up
> with off-by-two when calculating free_pages.
>
> Spotted-by: Wang Sheng-Hui<shhuiw@gmail.com>
> Signed-off-by: Michal Hocko<mhocko@suse.cz>
> ---
> mm/page_alloc.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9dd443d..8a2f1b6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
> long min = mark;
> int o;
>
> - free_pages -= (1<< order) + 1;
> + free_pages -= (1<< order) - 1;
> if (alloc_flags& ALLOC_HIGH)
> min -= min / 2;
> if (alloc_flags& ALLOC_HARDER)
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
--
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^ permalink raw reply [flat|nested] 6+ messages in thread