linux-kernel - Re: Pagecache: find_or_create_page does not call a proper page allocator function

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Date:	Tue, 24 Apr 2007 13:20:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org, pj@....com
Subject: Re: Pagecache: find_or_create_page does not call a proper page
 allocator function

On Tue, 24 Apr 2007 13:12:42 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Tue, 24 Apr 2007, Andrew Morton wrote:
> 
> > > A highmem page can have buffers???
> > 
> > yep.  Take a 4k page which is stored in four discontiguous 1k disk blocks. The
> > data at page_buffers(page) is the sole way in which we track which parts of
> > the page belong to which blocks of the disk.
> 
> But I see no use of kmap for buffer access? The data is not accessible.
> 

The kernel rarely has a need to actually read or write the page's contents
with the CPU.  On those occasions where it does, it will use kmap.  Search
for zero_user_page() and kmap in rc7-mm1's fs/buffer.c

But that's file pagecache, which can be in highmem.  File metadata is accessed
within the filesystems without kmapping.  This:

box:/usr/src/linux-2.6.21-rc7> grep '[-]>b_data' fs/*/*.c | wc -l 
1017

explains why that never got fixed.
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