linux-kernel - Re: [PATCH] fbdefio: add set_page

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Date:	Wed, 20 Aug 2008 08:27:33 -0400
From:	Markus Armbruster <armbru@...hat.com>
To:	"Jaya Kumar" <jayakumar.lkml@...il.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ian Campbell" <ijc@...lion.org.uk>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org, "Nick Piggin" <npiggin@...e.de>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Hugh Dickins" <hugh@...itas.com>,
	"Johannes Weiner" <hannes@...urebad.de>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Kel Modderman" <kel@...ku42.de>
Subject: Re: [PATCH] fbdefio: add set_page_dirty handler to deferred IO FB

"Jaya Kumar" <jayakumar.lkml@...il.com> writes:

> On Tue, Aug 19, 2008 at 2:38 AM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Tue, 19 Aug 2008 07:02:45 +0100 Ian Campbell <ijc@...lion.org.uk> wrote:
>>
>>> +static int fb_deferred_io_set_page_dirty(struct page *page)
>>> +{
>>> +     if (!PageDirty(page))
>>> +             SetPageDirty(page);
>>> +     return 0;
>>> +}
>>
>> <searches, finds the thread.  "kernel BUG at lib/radix-tree.c:473!">
>>
>> Is there actually any benefit in setting these pages dirty?  Or should
>> this be an empty function?  I see claims in the above thread that this
>> driver uses PG_dirty for optimising writeback but I can't immediately
>> locate any code which actually does that.
>
> Hi Andrew,
>
> I hope I have understood your question. You are right that PG_dirty
> isn't used directly in defio. The defio portion does use each
> page_mkwrite callback to build a list of the pages of the framebuffer
> that were written to and then passes that list to pvfb (in this case).
> pvfb then optimizes writeback by interpreting that list according to
> its framebuffer and sending it to its actual destination. I think
> Markus's code in xenfb_deferred_io() and xenfb_send* is doing the
> latter.

Exactly.  xenfb_deferred_io() is the callback that receives the list
of pages that have been dirtied from fb_deferred_io_work().  It
computes a rectangle covering all these pages, and passes it to
xenfb_refresh().  Which takes care of sending it to the backend.

[...]
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