linux-kernel - Re: [patch 1/3] mm: introduce follow

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Date:	Fri, 8 May 2009 17:51:24 +0900
From:	Magnus Damm <magnus.damm@...il.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-media@...r.kernel.org, hverkuil@...all.nl,
	lethal@...ux-sh.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] mm: introduce follow_pte()

On Wed, May 6, 2009 at 6:21 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> On Tue, May 05, 2009 at 02:05:17PM -0700, Andrew Morton wrote:
>> On Tue, 5 May 2009 22:38:07 +0200
>> Johannes Weiner <hannes@...xchg.org> wrote:
>> > On Tue, May 05, 2009 at 12:24:42PM -0700, Andrew Morton wrote:
>> > > On Mon,  4 May 2009 11:54:32 +0200
>> > > Johannes Weiner <hannes@...xchg.org> wrote:
>> > >
>> > > > A generic readonly page table lookup helper to map an address space
>> > > > and an address from it to a pte.
>> > >
>> > > umm, OK.
>> > >
>> > > Is there actually some point to these three patches?  If so, what is it?
>> >
>> > Magnus needs to check for physical contiguity of a VMAs backing pages
>> > to support zero-copy exportation of video data to userspace.
>> >
>> > This series implements follow_pfn() so he can walk the VMA backing
>> > pages and ensure their PFNs are in linear order.
>> >
>> > [ This patch can be collapsed with 2/3, I just thought it would be
>> >   easier to read the diffs when having them separate. ]
>> >
>> > 1/3 and 2/3: factor out the page table walk from follow_phys() into
>> > follow_pte().
>> >
>> > 3/3: implement follow_pfn() on top of follow_pte().
>>
>> So we could bundle these patches with Magnus's patchset, or we could
>> consider these three patches as a cleanup or something.
>>
>> Given that 3/3 introduces an unused function, I'm inclined to sit tight
>> and await Magnus's work.
>
> Yeah, I didn't see the video guys responding on Magnus' patch yet, so
> let's wait for them.
>
> Magnus, the actual conversion of your code should be trivial, could
> you respin it on top of these three patches using follow_pfn() then?

So I tested the patches in -mm (1/3, 2/3, 3/3) together with the zero
copy patch and everything seems fine. Feel free to add acks from me,
least for patch 1/3 and 3/3 - i know too little about the generic case
to say anything about 2/3.

Acked-by: Magnus Damm <damm@...l.co.jp>

I'll send V3 of my zero copy patch in a little while. Thanks a lot for the help!

Cheers,

/ magnus
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