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Date: Tue, 24 Apr 2007 10:51:47 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hugh@veritas.com>
Cc: Christoph Lameter <clameter@sgi.com>, Nick Piggin <npiggin@suse.de>,
       linux-kernel@vger.kernel.org, pj@sgi.com
Subject: Re: Pagecache: find_or_create_page does not call a proper page
 allocator function
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On Tue, 24 Apr 2007 18:45:03 +0100 (BST) Hugh Dickins <hugh@veritas.com> wrote:

> On Tue, 24 Apr 2007, Christoph Lameter wrote:
> > On Tue, 24 Apr 2007, Hugh Dickins wrote:
> > 
> > > I've not yet looked at the patch under discussion, but this remark
> > > prompts me...  a couple of days ago I got very worried by the various
> > > hard-wired GFP_HIGHUSER allocations in mm/migrate.c and mm/mempolicy.c,
> > > and wondered how those would work out if someone has a blockdev mmap'ed.
> > 
> > Hmmm.... These not that critical given that 32 bit NUMA systems are a bit 
> > rare.
> 
> That's true.  And everybody but the owners of those systems wish
> fervently that they didn't exist ;)
> 
> > And if a page is in the wrong area then it can be bounced before I/O 
> > is performed on it.
> 
> I think that much is also true, but not where the problem lies.
> Isn't the problem that filesystems using these block devices
> expect their metadata to be accessible without kmap calls?
> 

yup.  wherever we dereference buffer_head.b_data we're touching
page_address(buffer_head.b_page) without kmapping.