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

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Message-Id: <1193738177.27652.69.camel@twins>
Date:	Tue, 30 Oct 2007 10:56:17 +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 21:22 -0400, Jaya Kumar wrote:
> On 10/29/07, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > [ 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.
> 
> :-) If it's anybody's fault, it's mine for not testing properly. My bad.
> 
> In the case of defio, I think it's no trouble to build a list of vmas
> at mmap time and then to iterate through them when it's ready for
> mkclean time as you suggested. I don't fully understand page->index
> yet. I had thought it was only used by swap cache or file map.
> 
> On an unrelated note, I was looking for somewhere to stuff a 16 bit
> offset (so that I have a cheap way to know which struct page
> corresponds to which framebuffer block or offset) for another driver.
> I had thought page->index was it but I think I am wrong now.

Yeah, page->index is used along with vma->vmpgoff and vma->vm_start to
determine the address of the page in the given vma:

  address = vma->vm_start + ((page->index - vma->vm_pgoff) << PAGE_SHIFT);

and from that address the pte can be found by walking the vma->vm_mm
page tables.

So page->index does what you want it to, identify which part of the
framebuffer this particular page belongs to.

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