linux-kernel - Re: [RFC/PATCH 1/3] SLAB: Add PageSlab checking to ksize()

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Date:	Thu, 22 May 2008 13:55:16 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Christoph Lameter <clameter@....com>, linux-kernel@...r.kernel.org,
	mpm@...enic.com, dhowells@...hat.com
Subject: Re: [RFC/PATCH 1/3] SLAB: Add PageSlab checking to ksize()

On Thu, May 22, 2008 at 07:43:02AM +0300, Pekka Enberg wrote:
> Paul Mundt wrote:
> >Shouldn't SLOB's PageSlab usage should mimic that of SLUB in this case
> >instead? PG_slab doesn't buy us much if we can already sort out the size
> >through compound_order(), it's the kmem_cache_alloc() and <= PAGE_SIZE
> >kmalloc()'s where __GFP_COMP isn't true and where PG_slab should be set.
> 
> Well, we should really be calling ksize() in the nommu case and although 
> Matt and Christoph don't seem to agree with me here, I'd much rather 
> have *all* allocators set PageSlab for all the pages they return (yes, 
> including pass-through ones) to get us back where we were with SLAB. We 
> currently don't have any means to see whether an arbitrary page is part 
> of the slab or not and I'd argue that's PageSlab is an established API 
> (that makes sense).
> 
> Furthermore, if ksize() in SLOB doesn't work for kmem_cache_alloc() 
> pages, I think it should be fixed as well.
> 
SLOB's kmem_cache_alloc() pages won't have PageSlab set, so ksize()
will never be called, and we will simply always return PAGE_SIZE from
kobjsize() for those objects, as they don't set __GFP_COMP either.
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