linux-kernel - Re: Weirdness in __alloc_bootmem_node

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Message-ID: <20120420191418.GA3569@merkur.ravnborg.org>
Date:	Fri, 20 Apr 2012 21:14:18 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Michal Hocko <mhocko@...e.cz>, yinghai@...nel.org,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Weirdness in __alloc_bootmem_node_high

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.

mips has this:

config ARCH_SPARSEMEM_ENABLE
        bool
        select SPARSEMEM_STATIC

So SPARSEMEM_VMEMMAP is not enabled.

__alloc_bootmem_node_high() is used in mm/sparse-vmemmap.c which
also depends on CONFIG_SPARSEMEM_VMEMMAP.


So I really do not see the logic in __alloc_bootmem_node_high()
being used anymore and it can be replaced by __alloc_bootmem_node()

	Sam
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