linux-kernel - Re: [PATCH v2] percpu, x86: don't use PMD_SIZE as embedded atom

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Date:	Fri, 27 Apr 2012 10:53:03 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	ShuoX Liu <shuox.liu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] percpu, x86: don't use PMD_SIZE as embedded
 atom_size on 32bit

On Fri, Apr 27, 2012 at 10:51:43AM -0700, H. Peter Anvin wrote:
> On 04/27/2012 10:39 AM, Tejun Heo wrote:
> > With the embed percpu first chunk allocator, x86 uses either PAGE_SIZE
> > or PMD_SIZE for atom_size.  PMD_SIZE is used when CPU supports PSE so
> > that percpu areas are aligned to PMD mappings and possibly allow using
> > PMD mappings in vmalloc areas in the future.  Using larger atom_size
> > doesn't waste actual memory; however, it does require larger vmalloc
> > space allocation later on for !first chunks.
> > 
> > With reasonably sized vmalloc area, PMD_SIZE shouldn't be a problem
> > but x86_32 at this point is anything but reasonable in terms of
> > address space and using larger atom_size reportedly leads to frequent
> > percpu allocation failures on certain setups.
> > 
> > As there is no reason to not use PMD_SIZE on x86_64 as vmalloc space
> > is aplenty and most x86_64 configurations support PSE, fix the issue
> > by always using PMD_SIZE on x86_64 and PAGE_SIZE on x86_32.
> > 
> > v2: drop cpu_has_pse test and make x86_64 always use PMD_SIZE and
> >     x86_32 PAGE_SIZE as suggested by hpa.
> > 
> > Signed-off-by: Tejun Heo <tj@...nel.org>
> > Reported-by: Yanmin Zhang <yanmin.zhang@...el.com>
> > Reported-by: ShuoX Liu <shuox.liu@...el.com>
> > Acked-by: H. Peter Anvin <hpa@...or.com>
> > LKML-Reference: <4F97BA98.6010001@...el.com>
> > Cc: stable@...r.kernel.org
> 
> You're carrying this, or should I?

I have another percpu fix to push so I can carry it.

Thanks.

-- 
tejun
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