linux-kernel - Re: [patch 1/7] x86 PAT: store vm_pgoff for all linear_over_vma_region mappings

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Date:	Thu, 18 Dec 2008 23:33:03 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hugh@...itas.com" <hugh@...itas.com>,
	"arjan@...radead.org" <arjan@...radead.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"rdreier@...co.com" <rdreier@...co.com>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: [patch 1/7] x86 PAT: store vm_pgoff for all linear_over_vma_region mappings - v3

On Thu, Dec 18, 2008 at 02:10:57PM -0800, Pallipadi, Venkatesh wrote:
> On Thu, Dec 18, 2008 at 01:27:28PM -0800, Nick Piggin wrote:
> > 
> > This is fine by me, however:
> > 1. Can you add some comments to say "this is not for core vm but for pat,
> >    oh and a pgoff of zero is not going to work".
> 
> OK. Will add comments about both the points.
> 
> > 2. Can you please justify to me (or the changelog) roughly why PAT wants
> >    to know if the mapping is linear or not? Presumably it has to handle
> >    both types? If performance wasn't an issue, then you could manually scan
> >    the ptes to verify (which would solve your zero-offset bug). etc.
> 
> The main reason is performance. If we know it is linear, we can track the entire
> region as one block and do the reserve free for entire region. But, if it is
> not linear, then we have to reserve memtype of physical addresses page by page.
> This will not be optimal as it will result in reserve and free becoming
> slower. Almost all users that we find in kernel today (atleast in x86)  are
> all linear.

OK, so it is not a bug to miss the zero pgoff case then. That's good
to know and should be added to comments.

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