linux-kernel - Re: [patch 2/2] only allow nonlinear vmas for ram backed filesystems

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Date:	Thu, 05 Apr 2007 11:50:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch 2/2] only allow nonlinear vmas for ram backed
	filesystems

On Thu, 2007-04-05 at 11:39 +0200, Miklos Szeredi wrote:
> > > +		/*
> > > +		 * page_mkclean doesn't work on nonlinear vmas, so if dirty
> > > +		 * pages need to be accounted, emulate with linear vmas.
> > > +		 */
> > > +		if (mapping_cap_account_dirty(mapping)) {
> > 
> > Perhaps this should read:
> > 
> > 		if (vma_wants_writenotify(vma)) {
> > 
> 
> I looked at that, but IIRC vma_wants_writenotify() doesn't work after
> mmap(), because of the updated protection bits.

Right, bother, that again. I fudged it in mprotect by setting the pgprot
bits to what was expected although I had a parametrised version earlier.
But that was disliked.

> > That way we would even allow read only non-linear mappings of 'real'
> > filesystem files.
> 
> Well, we could do that, but is it really worth the hassle?  The real
> question is whether anyone would want to use non-linear
> shared-read-only mappings or not.

Hmm, yeah, I thought that was the case with that code snippet Andrew
pulled of the interweb, but on second inspection they do map it writable
too. I was led astray by the fact that they map the same file twice.

Oh well, lets just keep the patch as is then.

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