linux-kernel - Re: [GIT PATCH] x86,percpu: fix pageattr handling with remap allocator

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Date:	Sat, 16 May 2009 12:09:24 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"JBeulich@...ell.com" <JBeulich@...ell.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel-owner@...r.kernel.org" 
	<linux-kernel-owner@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PATCH] x86,percpu: fix pageattr handling with remap
 allocator

On Sat, 2009-05-16 at 08:16 -0700, Tejun Heo wrote:
> Hello, Suresh.
> 
> Suresh Siddha wrote:
> > Tejun, Can you please educate me why we need to map this first
> > percpu chunk (which is pre-allocated during boot and is physically
> > contiguous) into vmalloc area?
> 
> To make areas for each cpu congruent such that the address offset of a
> percpu symbol for CPU N is always the same from the address for CPU 0.

But for the first percpu chunk, isn't it the case that the physical
address allocations for a particular cpu is contiguous (as you are using
one bootmem allocation for whole PMD_SIZE for any given cpu)? So both
the kernel direct mapping aswell as the vmalloc mappings are contiguous
for the first chunk, on any given cpu. Right?

> > Perhaps even for the other dynamically allocated secondary chunks?
> > (as far as I can see, all the chunk allocations seems to be
> > physically contiguous and later mapped into vmalloc area)..
> > 
> > That should simplify these things quite a bit(atleast for first
> > percpu chunk).  I am missing something obvious I guess.
> 
> Hmm... Sorry I don't really follow.  Can you please elaborate the
> question?

For the first percpu chunk, we can use the kernel direct mapping and
avoid the vmalloc mapping of PMD_SIZE. And avoid the vmap address
aliasing problem (wrt to free pages that we have given back to -mm) that
we are trying to avoid with this patchset (as the existing cpa code
already takes care of the kernel direct mappings).

thanks,
suresh

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