linux-kernel - Re: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data can be mapped directly

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Date:	Fri, 25 Sep 2009 19:05:28 +0800
From:	graff yang <graff.yang@...il.com>
To:	David Howells <dhowells@...hat.com>
Cc:	torvalds@...l.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Paul Mundt <lethal@...ux-sh.org>, Mel Gorman <mel@....ul.ie>,
	Greg Ungerer <gerg@...pgear.com>
Subject: Re: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data 
	can be mapped directly

On Fri, Sep 25, 2009 at 4:24 PM, David Howells <dhowells@...hat.com> wrote:
> graff yang <graff.yang@...il.com> wrote:
>
>> > +       if (capabilities & BDI_CAP_MAP_DIRECT) {
>>
>> This will breaks many drivers, e.g. some frame-buffer drivers, on NOMMU
>> system.  Because they don't have get_unmapped_area().  These drivers depend
>> on it's mmap() to return the frame-buffer base address.
>
> Then they won't work on NOMMU.  Read Documentation/nommu-mmap.txt:
>
>        ============================================
>        PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
>        ============================================
>
>        To provide shareable character device support, a driver must provide a
>        file->f_op->get_unmapped_area() operation. The mmap() routines will
>        call this to get a proposed address for the mapping. This may return an
>        error if it doesn't wish to honour the mapping because it's too long,
>        at a weird offset, under some unsupported combination of flags or
>        whatever.
>
> The chardev driver doesn't provide the virtual address through its mmap() fop
> in MMU-mode either - that's provided by do_mmap_pgoff().
>
> David
>
OK, I think do_mmap_pgoff() now looks nice with your patch.   Merely,
at least the alsa/fb driver
will crash due to the vm_region changes.
The alsa guys are fixing this issue:
http://mailman.alsa-project.org/pipermail/alsa-devel/2009-March/015802.html

Is there any other things be affected?

-- 
-Graff
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