linux-kernel - Re: [PATCH 1/2] NOMMU: Set BDI capabilities for /dev/mem and /dev/kmem

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Date:	Wed, 30 Aug 2006 10:24:55 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	linux-kernel@...r.kernel.org, uclinux-dev@...inux.org
Subject: Re: [PATCH 1/2] NOMMU: Set BDI capabilities for /dev/mem and /dev/kmem 

Andrew Morton <akpm@...l.org> wrote:

> Perhaps one could make mem_bdi==NULL if !CONFIG_MMU, for a minor space
> saving.

The problem is that takes the #ifdef-avoidance scheme a step too far.  If we
do that, the kernel will crash.  Observe the following:

  Breakpoint 1, memory_open (inode=0xc0f7da04, filp=0xc09d8ba0) at fs.h:635
  635		return MINOR(inode->i_rdev);
  (gdb) n
  634	{
  (gdb) 
  904				filp->f_mapping->backing_dev_info = &mem_bdi;
  (gdb) i sym filp->f_mapping->backing_dev_info
  default_backing_dev_info in section .data

Note how the BDI pointer is already set to the default which we then override.
We either need to fill in mem_bdi correctly for MMU or only override the BDI
pointer if !MMU.

Of course, that doesn't touch on the matter of the compiler not letting you do
&NULL, though I assume you meant make &mem_bdi == NULL.

Also, the default BDI is incorrect for /dev/mem and /dev/kmem since it does
not permit shared mappings, so I think we need mem_bdi anyway, though I should
perhaps make it more general, so that it can apply to all mappable chardevs.

David
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