linux-kernel - Re: 2.6.22 -mm merge plans: slub

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Date:	Wed, 2 May 2007 10:01:41 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Hugh Dickins <hugh@...itas.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: 2.6.22 -mm merge plans: slub

On Wed, 2 May 2007, Hugh Dickins wrote:

> > Why would we need to go back to SLAB if we have not switched to SLUB? SLUB 
> > is marked experimental and not the default.
> 
> I said above that I thought SLUB ought to be defaulted to on throughout
> the -rcs: if we don't do that, we're not going to learn much from having
> it in Linus' tree.

I'd rather be careful with that..... mm is enough for now. Why go to the 
extremes immediately. If it is an option then people can gradually start 
testing with it.

> > The only problems that I am aware of is(or was) the issue with arches 
> > modifying page struct fields of slab pages that SLUB needs for its own 
> > operations. And I thought it was all fixed since the powerpc guys were 
> > quiet and the patch was in for i386.
> 
> You're forgetting your unions in struct page: in the SPLIT_PTLOCK
> case (NR_CPUS >= 4) the pagetable code is using spinlock_t ptl,
> which overlays SLUB's first_page and slab pointers.

Uhhh.... Right. So SLUB wont work if the lowest page table block is 
managed via slabs.

> I just tried rebuilding powerpc with the SPLIT_PTLOCK cutover
> edited to 8 cpus instead, and then no crash.
> 
> I presume the answer is just to extend your quicklist work to
> powerpc's lowest level of pagetables.  The only other architecture

I am not sure how PowerPCs lower pagetable pages work. If they are of 
PAGE_SIZE then this is no problem.

> which is using kmem_cache for them is arm26, which has
> "#error SMP is not supported", so won't be giving this problem.

Ahh. Good.

But these are arch specific problems. We could use 
ARCH_USES_SLAB_PAGE_STRUCT to disable SLUB on these platforms.
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