netdev - [PATCH] slub: Make CONFIG_PAGE_ALLOC work with new fastpath

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Date:	Fri, 13 May 2011 16:15:41 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Vegard Nossum <vegardno@....uio.no>,
	Pekka Enberg <penberg@...helsinki.fi>,
	casteyde.christian@...e.fr,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org
Subject: [PATCH] slub: Make CONFIG_PAGE_ALLOC work with new fastpath

I ran this with hackbench under KVM with CONFIG_DEBUG_PAGEALLOC and I saw
no failures.


Subject: slub: Make CONFIG_PAGE_ALLOC work with new fastpath

Fastpath can do a speculative access to a page that CONFIG_PAGE_ALLOC may have
marked as invalid to retrieve the pointer to the next free object.

Use probe_kernel_read in that case in order not to cause a page fault.

Signed-off-by: Christoph Lameter <cl@...ux.com>
---
 mm/slub.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2011-05-10 14:31:28.000000000 -0500
+++ linux-2.6/mm/slub.c	2011-05-10 14:31:35.000000000 -0500
@@ -261,6 +261,18 @@ static inline void *get_freepointer(stru
 	return *(void **)(object + s->offset);
 }

+static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
+{
+	void *p;
+
+#ifdef CONFIG_DEBUG_PAGEALLOC
+	probe_kernel_read(&p, (void **)(object + s->offset), sizeof(p));
+#else
+	p = get_freepointer(s, object);
+#endif
+	return p;
+}
+
 static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
 {
 	*(void **)(object + s->offset) = fp;
@@ -1933,7 +1945,7 @@ redo:
 		if (unlikely(!irqsafe_cpu_cmpxchg_double(
 				s->cpu_slab->freelist, s->cpu_slab->tid,
 				object, tid,
-				get_freepointer(s, object), next_tid(tid)))) {
+				get_freepointer_safe(s, object), next_tid(tid)))) {

 			note_cmpxchg_failure("slab_alloc", s, tid);
 			goto redo;
--
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