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 13:10:42 -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 12:59:17 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Tue, 24 Apr 2007, Andrew Morton wrote:
> 
> > No, think of the following scenario:
> > 
> > - file I/O causes a read of an ext2 file's bitmap.  The bitmap is
> >   brought into /dev/hda1's pagecache using !__GFP_HIGHMEM
> > 
> > - references are released against that page and it's now just clean
> >   reclaimable pagecache
> > 
> > - someone (say, an online filesystem checker or something) mmaps
> >   /dev/hda1 and reads that page.
> > 
> > - migration comes alnog and migrates that page into highmem
> > 
> > - file I/O causes a read of that bitmap again.  We find it in
> >   /dev/hda's pagecache.
> 
> Read of the bitmap? How would that work? Page cache lookup right?

yup.

	sb_bread
	->__bread
	  ->__getblk
	    ->__find_get_block
	      ->__find_get_block_slow
	        ->find_get_page

> >   Here's set_bh_page().
> 
> 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.

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