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Subject[PATCH] Revert "slub: Remove node check in slab_free"

* Ingo Molnar <mingo@elte.hu> wrote:

>
> * James Morris <jmorris@namei.org> wrote:
>
> > I started experiencing a boot hang in 2.6.39+, and bisected it to this
> > change:
>
> i too was seeing various instabilities and SLUB corruption in -tip testing.
> Every 5th randconfig kernel crashes.
>
> I started testing the revert.

Stability of -tip has increased dramatically (36 test iterations and
no crash), so yes this revert resolves the crashes.

I've attached the changelogified revert patch below. James, since i
was seeing these sporadically you've saved me a couple of hours of
rather painful bisection work! :-)

Thanks,

Ingo

----------------->
From 6ac0730862b6dd1b45bc86e8e61e4026293b09f9 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@elte.hu>
Date: Wed, 25 May 2011 09:37:47 +0200
Subject: [PATCH] Revert "slub: Remove node check in slab_free"
This reverts commit 442b06bcea23a01934d3da7ec5898fa154a6cafb.

As Linus explained it's broken. Quoting Linus:

That whole "deactivate_slab()" + "c->page = NULL" that this patch
does looks bogus.

Look at __slab_alloc(), we have:

page = c->page;
if (!page)
goto new_slab;
slab_lock(page);
if (unlikely(!node_match(c, node)))
goto another_slab;
and let's assume we have two users racing on that "c->page". The
"slab_lock()" is going to work for one of them, right?

Ok, so the one it works for will then hit:

if (kmem_cache_debug(s))
goto debug;
and thus get to the new "deactivate_slab(s,c) + c->page = NULL" and
then unlock the page.

In the meantime, the one that wasn't able to lock the page will now
go forward, but will not have "node_match()" any more, so it does
that "goto another_slab".

Which does "deactivate_slab(s,c)" again, and now c->page is NULL, so
that totally breaks.

Reported-and-bisected-by: James Morris <jmorris@namei.org>
Analyzed-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/alpine.LRH.2.00.1105242146240.12134@tundra.namei.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
mm/slub.c | 4 +---
1 files changed, 1 insertions(+), 3 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 4ea7f1a..ed1281b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1881,8 +1881,6 @@ debug:

page->inuse++;
page->freelist = get_freepointer(s, object);
- deactivate_slab(s, c);
- c->page = NULL;
c->node = NUMA_NO_NODE;
goto unlock_out;
}
@@ -2114,7 +2112,7 @@ redo:
tid = c->tid;
barrier();

- if (likely(page == c->page)) {
+ if (likely(page == c->page && c->node != NUMA_NO_NODE)) {
set_freepointer(s, object, c->freelist);

if (unlikely(!irqsafe_cpu_cmpxchg_double(

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