Re: [PATCH 1/3] mm, nobootmem: fix wrong usage of max_low_pfn

From: Joonsoo Kim
Date: Tue Mar 19 2013 - 02:42:38 EST


On Tue, Mar 19, 2013 at 03:25:22PM +0900, Joonsoo Kim wrote:
> 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@xxxxxxx> 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@xxxxxxx>
> > >
> > > 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.

How about using "memblock.current_limit"?

unsigned long end_pfn = min_t(unsigned long, PFN_DOWN(end),
memblock.current_limit);

Thanks.

>
> >
> > Thanks
> >
> > Yinghai
> >
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