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

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Message-ID: <Pine.LNX.4.64.0711031708040.10266@blonde.wat.veritas.com>
Date:	Sat, 3 Nov 2007 17:10:54 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] slub: fix leakage

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

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 c->freelist is empty,
and will lose all of its free slots when c->page->freelist is not empty.
Instead just do a local allocation from c->freelist when it has one.

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

Signed-off-by: Hugh Dickins <hugh@...itas.com>
---
I recommend this for 2.6.24-rc2 and 2.6.23-stable.

 mm/slub.c |    5 +++++
 1 file changed, 5 insertions(+)

--- 2.6.24-rc1/mm/slub.c	2007-10-24 07:16:04.000000000 +0100
+++ linux/mm/slub.c	2007-11-03 13:22:31.000000000 +0000
@@ -1525,6 +1525,11 @@ new_slab:
 				 * want the current one since its cache hot
 				 */
 				discard_slab(s, new);
+				if (c->freelist) {
+					object = c->freelist;
+					c->freelist = object[c->offset];
+					return object;
+				}
 				slab_lock(c->page);
 				goto load_freelist;
 			}
-
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