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 09:24:27 +0100
From:	David Howells <dhowells@...hat.com>
To:	graff yang <graff.yang@...il.com>
Cc:	dhowells@...hat.com, 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

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
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