Re: [PATCH 1/5] Forking ext4 filesystem from ext3 filesystem

On Thu, 2006-08-10 at 12:23 -0700, Andrew Morton wrote:
> On Thu, 10 Aug 2006 11:51:34 -0700
> Badari Pulavarty <[email protected]> wrote:
> 
> > Andrew Morton wrote:
> > > Also, JBD is presently feeding into submit_bh() buffer_heads which span two
> > > machine pages, and some device drivers spit the dummy.  It'd be better to
> > > fix that once, rather than twice..  
> > >   
> > Andrew,
> > 
> > I looked at this few days ago. I am not sure how we end up having 
> > multiple pages (especially,
> > why we end up having buffers with bh_size > pagesize) ? Do you know why ?
> > 
> 
> It's one or both of the jbd_kmalloc(bh->b_size) calls in
> fs/jbd/transaction.c.  Here we're allocating data to attach to a bh which
> later gets fed into submit_bh().
> 
> Problem is, with CONFIG_DEBUG_SLAB=y, the data which kmalloc() returns can
> be offset by 4 bytes due to redzoning.
> 
> Example: if the fs is using a 1k blocksize and we have a 4k pagesize, the
> data coming back from kmalloc may have an address of 0xnnnnxc04, so the
> data which we later feed into submit_bh() will span two pages.
> 
> A simple fix would be to replace kmalloc() with a call to alloc_page(). 
> We'd need to work out how much memory that will worst-case-waste.  If "not
> much" then OK.
> 
> If "quite a lot in the worst case" then we'd need something more elaborate.

Would some like this be okay:

#ifdef CONFIG_DEBUG_SLAB
	return alloc_page(...
#else
	return kmalloc(...
#endif

This keeps it simple, and should be still be efficient in the
non-DEBUG_SLAB case.

>  I'd suggest that ext3 implement ext3-private slab caches of size 1024,
> 2048, 4096 and perhaps 8192.  Those caches should be kmem_cache_create()d
> on-demand at mount-time.  They should be created with appropriate slab
> options to defeat the redzoning.  The transaction.c code should use the
> appropriate slab (based on b_size) rather than using kmalloc().  The
> up-to-four private slab caches should be destroyed on ext3 rmmod.
-- 
David Kleikamp
IBM Linux Technology Center


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