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Date: Tue, 1 Aug 2006 10:36:55 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
To: Andrew Morton <akpm@osdl.org>
cc: Herbert Xu <herbert@gondor.apana.org.au>, linux-kernel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net
Subject: Re: [BLOCK] bh: Ensure bh fits within a page
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On Mon, 31 Jul 2006, Andrew Morton wrote:

> > Sure, this particular instance is in journal_write_metadata_buffer
> > where the bh may be constructed from kmalloc memory (search for the
> > call to jbd_rep_kmalloc).  Because the memory returned by kmalloc
> > may straddle a page (when slab debugging is enabled that is), this
> > causes a broken bh to be injected into submit_bh.
> > 
> 
> Crap, that's hard to fix.   Am I allowed to blame submit_bh()? ;)
> 
> uhm, we don't want to lose kmalloc redzoning, so I guess we need to create
> on-demand ext3-private slab caches for 1024, 2048, and 4096 bytes.  With
> the appropriate slab flags to defeat the redzoning.

The slab allocator gives no guarantee that a structure is not straddling a 
page boundary regardless of debug or not. It may just happen that the 
objects are arranged if kmem_cache_cretae() is called with certain 
parameters. Another arch with other cacheline alignment and another page 
size may arrange the objects differently.