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

[Wang Sheng-Hui <shhuiw@xxxxxxxxx>]

Sorry, Michal.

2011/12/6 Michal Hocko <mhocko@xxxxxxx>

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@xxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 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