linux-kernel - Re: Weirdness in __alloc_bootmem_node

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Date:	Fri, 20 Apr 2012 21:29:37 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Tejun Heo <tj@...nel.org>, yinghai@...nel.org, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Weirdness in __alloc_bootmem_node_high

On Fri 20-04-12 21:14:18, Sam Ravnborg wrote:
> On Fri, Apr 20, 2012 at 11:29:07AM -0700, Tejun Heo wrote:
> > On Tue, Apr 17, 2012 at 05:55:02PM +0200, Michal Hocko wrote:
> > > Hi,
> > > I just come across the following condition in __alloc_bootmem_node_high
> > > which I have hard times to understand. I guess it is a bug and we need
> > > something like the following. But, to be honest, I have no idea why we
> > > care about those 128MB above MAX_DMA32_PFN.
> > > ---
> > >  mm/bootmem.c |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/bootmem.c b/mm/bootmem.c
> > > index 0131170..5adb072 100644
> > > --- a/mm/bootmem.c
> > > +++ b/mm/bootmem.c
> > > @@ -737,7 +737,7 @@ void * __init __alloc_bootmem_node_high(pg_data_t *pgdat, unsigned long size,
> > >  	/* update goal according ...MAX_DMA32_PFN */
> > >  	end_pfn = pgdat->node_start_pfn + pgdat->node_spanned_pages;
> > >  
> > > -	if (end_pfn > MAX_DMA32_PFN + (128 >> (20 - PAGE_SHIFT)) &&
> > > +	if (end_pfn > MAX_DMA32_PFN + (128 << (20 - PAGE_SHIFT)) &&
> > >  	    (goal >> PAGE_SHIFT) < MAX_DMA32_PFN) {
> > >  		void *ptr;
> > >  		unsigned long new_goal;
> > 
> > Regardless of x86 not using it, this is a bug fix and this code path
> > seems to be used by mips at least.
> 
> I took a quick look at this.
> __alloc_bootmem_node_high() is used in mm/sparse.c - but only
> if SPARSEMEM_VMEMMAP is enabled.

This is what I can see in the current (Linus) git:
./arch/sparc/Kconfig:   select SPARSEMEM_VMEMMAP_ENABLE
./arch/powerpc/Kconfig: select SPARSEMEM_VMEMMAP_ENABLE
./arch/ia64/Kconfig:    select SPARSEMEM_VMEMMAP_ENABLE
./arch/s390/Kconfig:    select SPARSEMEM_VMEMMAP_ENABLE
./arch/s390/Kconfig:    select SPARSEMEM_VMEMMAP
./arch/x86/Kconfig:     select SPARSEMEM_VMEMMAP_ENABLE if X86_64

So there are more arches which enable SPARSEMEM_VMEMMAP so the function
is used. Or am I missing something?

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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