Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?

Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?

[Michal Hocko]

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)
-- 
1.7.7.3

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?

[Mel Gorman]

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

Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages?

[KOSAKI Motohiro]

(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>