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

[Andrew Morton <akpm@osdl.org>]

On Thu, 10 Aug 2006 11:51:34 -0700
Badari Pulavarty <pbadari@us.ibm.com> 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.
 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.