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

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Date:	Wed, 2 May 2007 13:45:33 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....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 Tue, 1 May 2007, Christoph Lameter wrote:
> On Tue, 1 May 2007, Hugh Dickins wrote:
> 
> > Yes, to me it does.  If it could be defaulted to on throughout the
> > -rcs, on every architecture, then I'd say that's "finishing work";
> > and we'd be safe knowing we could go back to slab in a hurry if
> > needed.  But it hasn't reached that stage yet, I think.
> 
> 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.

And perhaps that line which appends "PREEMPT " to an oops report ought
to append "SLUB " too, for so long as there's a choice.

> 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.

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

Hugh
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