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

[Wang Sheng-Hui <shhuiw@gmail.com>]

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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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