linux-kernel - Re: vm_ops.page_mkwrite() fails with vmalloc on 2.6.23

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Message-Id: <1193696211.5644.100.camel@lappy>
Date:	Mon, 29 Oct 2007 23:16:50 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jaya Kumar <jayakumar.lkml@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, stefani@...bold.net,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: vm_ops.page_mkwrite() fails with vmalloc on 2.6.23


On Mon, 2007-10-29 at 19:17 +0100, Peter Zijlstra wrote:
> On Mon, 2007-10-29 at 13:51 -0400, Jaya Kumar wrote:
> > On 10/29/07, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Mon, 2007-10-29 at 01:17 -0700, Jaya Kumar wrote:
> > > > An aside, I just tested that deferred IO works fine on 2.6.22.10/pxa255.
> > > >
> > > > I understood from the thread that PeterZ is looking into page_mkclean
> > > > changes which I guess went into 2.6.23. I'm also happy to help in any
> > > > way if the way we're doing fb_defio needs to change.
> > >
> > > OK, seems I can't read. Or at least, I missed a large part of the
> > > problem.
> > >
> > > page_mkclean() hasn't changed, it was ->page_mkwrite() that changed. And
> > > looking at the fb_defio code, I'm not sure I understand how its
> > > page_mkclean() use could ever have worked.
> > >
> > > The proposed patch [1] only fixes the issue of ->page_mkwrite() on
> > > vmalloc()'ed memory. Not page_mkclean(), and that has never worked from
> > > what I can make of it.
> > >
> > > Jaya, could you shed some light on this? I presume you had your display
> > > working.
> > >
> > 
> > I thought I had it working. I saw the display update after each
> > mmap/write sequence to the framebuffer. I need to check if there's an
> > munmap or anything else going on in between write sequences that would
> > cause it to behave like page_mkclean was working.
> > 
> > Is it correct to assume that page_mkclean should mark the pages
> > read-only so that the next write would again trigger mkwrite?
> 
> Well, yes, that is the intended behaviour.
> 
> >  Even if the page was from a vmalloc_to_page()?
> 
> That is the crux, I only ever implemented it for file pages.

Hmm, so these vmalloc pages are mapped into user-space with
remap_pfn_range(), which doesn't have any form of rmap. That is, given a
pfn there is no way to obtain all ptes for it. So the interface to
page_mkclean() could never work for these (as it only provides a struct
page *).

[ also, remap_vmalloc_range() suffers similar issues, only file and anon
  have proper rmap ]

I'm not sure we want full rmap for remap_pfn/vmalloc_range, but perhaps
we could assist drivers in maintaining and using vma lists.

I think page_mkclean_one() would work if you'd manually set page->index
and iterate the vmas yourself. Although atm I'm not sure of anything so
don't pin me on it.

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