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

From: Matt Mackall
Date: Thu May 22 2008 - 11:01:53 EST

On Thu, 2008-05-22 at 07:43 +0300, Pekka Enberg wrote:
> Hi Paul,
> 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.

As I've said several times, ksize() on kmem_cache_alloced objects
-cannot work- on SLOB. Calling ksize() on something returned by
kmem_cache_alloc is a categorical error.

Mathematics is the supreme nostalgia of our time.

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