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

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Date:	Tue, 19 Mar 2013 15:25:22 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Yinghai Lu <yinghai@...nel.org>
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 10:47:41PM -0700, Yinghai Lu wrote:
> 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 ...

Below is queote from Russell King in 'https://lkml.org/lkml/2013/3/13/123'


 Now, max_low_pfn is initialized this way:
 
 /**
  * init_bootmem - register boot memory
  * @start: pfn where the bitmap is to be placed
  * @pages: number of available physical pages
  *
  * Returns the number of bytes needed to hold the bitmap.
  */
 unsigned long __init init_bootmem(unsigned long start, unsigned long pages)
 {
        max_low_pfn = pages;
        min_low_pfn = start;
        return init_bootmem_core(NODE_DATA(0)->bdata, start, 0, pages);
 }
 So, min_low_pfn is the PFN offset of the start of physical memory (so
 3GB >> PAGE_SHIFT) and max_low_pfn ends up being the number of pages,
 _not_ the maximum PFN value

So, if physical address doesn't start at 0, max_low_pfn doesn't represent 
the maximum PFN value. This is a case for ARM.

> 
> #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?

My implementation is totally wrong. :)
min_low_pfn is not proper value for this purpose.

I will fix it.
Sorry for noise.

Thanks.

> 
> Thanks
> 
> Yinghai
> 
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