linux-kernel - Re: [PATCH 1/2] slub: fix leakage

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Date:	Sat, 3 Nov 2007 18:52:10 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Olivér Pintér <oliver.pntr@...il.com>
cc:	Christoph Lameter <clameter@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
	stable@...nel.org
Subject: Re: [PATCH 1/2] slub: fix leakage

On Sat, 3 Nov 2007, Hugh Dickins wrote:
> On Sat, 3 Nov 2007, Olivér Pintér wrote:
> > Q: It's needed auch to 2.6.22-stable?
> 
> I guess so: though SLUB wasn't on by default in 2.6.22; and it being
> only a slow leak rather than a corruption, I was less inclined to
> agitate about it for releases further back.
> 
> But your question makes me realize I never even looked at 2.6.23 or
> 2.6.22 hereabouts, just assumed they were the same; let alone patch
> or build or test them.  The patches reject as such because quite a
> lot has changed around (there was no struct kmem_cache_cpu in either).
> 
> A hurried look suggests that the leakage problem was there in both,
> but let's wait to hear Christoph's expert opinion.

Okay, here's a version for 2.6.23 and 2.6.22...
Christoph, you've now Acked the 2.6.24 one, thanks:
do you agree this patch below should go to -stable?

Slub has been quite leaky under load.  Taking mm_struct as an example, in
a loop of swapping kernel builds, after the first iteration slabinfo shows:
Name        Objects Objsize    Space Slabs/Part/Cpu  O/S O %Fr %Ef Flg
mm_struct        55     840    73.7K         18/7/4    4 0  38  62 A
but Objects and Partials steadily creep up - after the 340th iteration:
mm_struct       110     840   188.4K        46/36/4    4 0  78  49 A
(example taken from 2.6.24-rc1: YMMV).

The culprit turns out to be __slab_alloc(), where it copes with the race
that another task has assigned the cpu slab while we were allocating one.
Don't rush off to load_freelist there: that assumes page->lockless_freelist
is empty, and will lose all its free slots when page->freelist is not empty.
Instead just do a local allocation from lockless_freelist when it has one.

Which fixes the leakage: Objects and Partials then remain stable.

Signed-off-by: Hugh Dickins <hugh@...itas.com>
---
Version of patch suitable and recommended for both 2.6.23-stable and
2.6.22-stable.  I've not run tests on either to observe the mounting
leakage; but a version of the patch below with a printk announcing
when non-empty freelist would overwrite non-empty lockless_freelist
does indeed show up in both (though notably less frequently than in
2.6.24-rc1 - something else seems to be making it more likely now).
But please wait for Christoph's Ack before committing to -stable.

 mm/slub.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- 2.6.23/mm/slub.c	2007-10-09 21:31:38.000000000 +0100
+++ linux/mm/slub.c	2007-11-03 18:23:07.000000000 +0000
@@ -1517,6 +1517,12 @@ new_slab:
 				 */
 				discard_slab(s, page);
 				page = s->cpu_slab[cpu];
+				if (page->lockless_freelist) {
+					object = page->lockless_freelist;
+					page->lockless_freelist =
+							object[page->offset];
+					return object;
+				}
 				slab_lock(page);
 				goto load_freelist;
 			}