linux-kernel - Re: [PATCH] Containment measures for slab objects on scatter gather lists

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Message-ID: <Pine.LNX.4.64.0706282346560.12146@schroedinger.engr.sgi.com>
Date:	Thu, 28 Jun 2007 23:50:27 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	David Miller <davem@...emloft.net>
cc:	akpm@...ux-foundation.org, hugh@...itas.com,
	James.Bottomley@...eleye.com, rmk+lkml@....linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Containment measures for slab objects on scatter gather
 lists

On Thu, 28 Jun 2007, David Miller wrote:

> From: Andrew Morton <akpm@...ux-foundation.org>
> Date: Thu, 28 Jun 2007 22:24:24 -0700
> 
> > So what happens when two quite different threads of control are doing
> > IO against two hunks of kmalloced memory which happen to come from the same
> > page?  Either some (kernel-wide) locking is needed, or that pageframe needs
> > to be treated as readonly?
> 
> Or you put an atomic_t at the beginning or tail of every SLAB
> object.  It's a space cost not a runtime cost for the common
> case which is:

Hmmm... We could do something like

kmem_cache_get(slab, object)

and

kmem_cache_put(slab, object)

kmem_cache_get would disable allocations from the slab and increment a 
refcount.

kmem_cache_put would enable allocations again if the refcount reaches 
one.

The problem is that freeing an object may cause writes to the object. F.e.
poisoning will overwrite the object on free. SLUB will put its free 
pointer in the first words etc.



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