linux-kernel - Re: [tip:x86/urgent] VM, x86, PAT: Change is_linear_pfn_mapping to not use vm

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Message-ID: <20090314025238.GA32065@wotan.suse.de>
Date:	Sat, 14 Mar 2009 03:52:38 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"elendil@...net.nl" <elendil@...net.nl>,
	"thellstrom@...are.com" <thellstrom@...are.com>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] VM, x86, PAT: Change is_linear_pfn_mapping to not use vm_pgoff

On Fri, Mar 13, 2009 at 10:00:07AM -0700, Pallipadi, Venkatesh wrote:
> On Fri, 2009-03-13 at 09:25 -0700, Nick Piggin wrote:
> > Why not just use another vm_flag for 2.6.29 and earlier (we have 2 left),
> > then do some flag rationalisation in 2.6.30?
> > 
> > But I do like just avoiding these games entirely and encoding it
> > explicitly.
> > 
> 
> Totally agree that this is ugly. But, I didn't like taking away one bit
> for such a light usage. VM_INSERTPAGE is hardly ever used in the kernel
> (wherever it is used, it is checked along with VM_PFNMAP).

That is a concern, but I think it makes the patch more intrusive
than a simple fix that might possibly introduce a bug. So for 2.6.29
we have spare flags so can use one of those for the fix.
 

> OK. I will resend the patch with taking away one of the available bits.
> But, I feel we need to do some multiplexing here, in a clean manner, for
> future.

That would always be appreciated. There are probably several flags that
can go away (eg. VM_RESERVED, VM_INSERTPAGE), others that can be merged
(eg nommu flags with mmu flags), others that can be one scalar value
rather than several binary values etc.

Also, if worst comes to worst, I think we could probably make vm_flags
64-bit on 32-bit compiles to get another 32 bits. Of course I'd rather
not until no other options left for reducing flags ;)
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