linux-kernel - Re: [PATCH] slab: deal with NULL pointers passed to kmem_cache

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Date:	Mon, 19 Mar 2007 14:41:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Pekka Enberg" <penberg@...helsinki.fi>
Cc:	mpm@...enic.com, "Christoph Lameter" <clameter@....com>,
	"ast@...dv.de" <ast@...dv.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slab: deal with NULL pointers passed to kmem_cache_free

On Mon, 19 Mar 2007 23:25:36 +0200 (EET)
"Pekka Enberg" <penberg@...helsinki.fi> wrote:

> 
> On 3/19/2007, "Andrew Morton" <akpm@...ux-foundation.org> wrote:
> > Would prefer to do:
> > 
> > static inline void kmem_cache_free_if_not_null(struct kmem_cache *cachep,
> > 						void *objp)
> > {
> > 	if (objp)
> > 		kmem_cache_free(cachep, objp);
> > }
> > 
> > so that we don't add extra overhead to all the thousands of existing,
> > well-behaved callsites.
> 
> That bloats kernel text all the same

But only for those callsites which choose to use it!  We avoid adding a
test-and-branch to those thousands of callsite which don't need it.

This is a super-hot path.

> so it's much cleaner to just make
> the callers explicitly check for NULL then. That said, I'm sorry but I
> just don't buy the "overhead" part of your argument since it's one
> branch and no extra data cache pressure especially as we're already
> doing the BUG_ON and page flag checking.

The BUG_ON (at least) should probably be moved into CONFIG_DEBUG_SLAB.

> But, since you're NAKing my patch, we need to get the mempool for from
> the original thread in to fix the oops.

We need to fix scsi rather than working around it in slab or in mempool -
it appears that it's getting its sg lists tangled up, and the problem has
been known since November (at least).
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