linux-kernel - Re: [PATCH 1/3] mm, nobootmem: fix wrong usage of max_low

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Date:	Mon, 18 Mar 2013 22:47:41 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Johannes Weiner <hannes@...xchg.org>,
	Jiang Liu <liuj97@...il.com>
Subject: Re: [PATCH 1/3] mm, nobootmem: fix wrong usage of max_low_pfn

On Mon, Mar 18, 2013 at 10:15 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> max_low_pfn reflect the number of _pages_ in the system,
> not the maximum PFN. You can easily find that fact in init_bootmem().
> So fix it.

I'm confused. for x86, we have max_low_pfn defined in ...

#ifdef CONFIG_X86_32
        /* max_low_pfn get updated here */
        find_low_pfn_range();
#else
        num_physpages = max_pfn;

        check_x2apic();

        /* How many end-of-memory variables you have, grandma! */
        /* need this before calling reserve_initrd */
        if (max_pfn > (1UL<<(32 - PAGE_SHIFT)))
                max_low_pfn = e820_end_of_low_ram_pfn();
        else
                max_low_pfn = max_pfn;

and under max_low_pfn is bootmem.

>
> Additionally, if 'start_pfn == end_pfn', we don't need to go futher,
> so change range check.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> diff --git a/mm/nobootmem.c b/mm/nobootmem.c
> index 5e07d36..4711e91 100644
> --- a/mm/nobootmem.c
> +++ b/mm/nobootmem.c
> @@ -110,9 +110,9 @@ static unsigned long __init __free_memory_core(phys_addr_t start,
>  {
>         unsigned long start_pfn = PFN_UP(start);
>         unsigned long end_pfn = min_t(unsigned long,
> -                                     PFN_DOWN(end), max_low_pfn);
> +                                     PFN_DOWN(end), min_low_pfn);

what is min_low_pfn ?  is it 0 for x86?

Thanks

Yinghai
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