slub: always align cpu_slab to honor cmpxchg_double requirement

On an architecture without CMPXCHG_LOCAL but with DEBUG_VM enabled,
the VM_BUG_ON() in __pcpu_double_call_return_bool() will cause an early
panic during boot unless we always align cpu_slab properly.

In principle we could remove the alignment-testing VM_BUG_ON() for
architectures that don't have CMPXCHG_LOCAL, but leaving it in means
that new code will tend not to break x86 even if it is introduced
on another platform, and it's low cost to require alignment.

Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 8b97308e65df3ccf094d79af23266e2da5fc73af..9ca008f0c542935440f3c39c0a5fb070dd6f5e8c 100644 (file)
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -259,6 +259,9 @@ extern void __bad_size_call_parameter(void);
  * Special handling for cmpxchg_double.  cmpxchg_double is passed two
  * percpu variables.  The first has to be aligned to a double word
  * boundary and the second has to follow directly thereafter.
+ * We enforce this on all architectures even if they don't support
+ * a double cmpxchg instruction, since it's a cheap requirement, and it
+ * avoids breaking the requirement for architectures with the instruction.
  */
 #define __pcpu_double_call_return_bool(stem, pcp1, pcp2, ...)          \
 ({                                                                     \

diff --git a/mm/slub.c b/mm/slub.c
index 7be0223531b090dbd9f716104714bc4d9ca9c7b5..35f351f26193a47145cd8bcb7f55ae091331ca97 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2320,16 +2320,12 @@ static inline int alloc_kmem_cache_cpus(struct kmem_cache *s)
        BUILD_BUG_ON(PERCPU_DYNAMIC_EARLY_SIZE <
                        SLUB_PAGE_SHIFT * sizeof(struct kmem_cache_cpu));
 
-#ifdef CONFIG_CMPXCHG_LOCAL
        /*
-        * Must align to double word boundary for the double cmpxchg instructions
-        * to work.
+        * Must align to double word boundary for the double cmpxchg
+        * instructions to work; see __pcpu_double_call_return_bool().
         */
-       s->cpu_slab = __alloc_percpu(sizeof(struct kmem_cache_cpu), 2 * sizeof(void *));
-#else
-       /* Regular alignment is sufficient */
-       s->cpu_slab = alloc_percpu(struct kmem_cache_cpu);
-#endif
+       s->cpu_slab = __alloc_percpu(sizeof(struct kmem_cache_cpu),
+                                    2 * sizeof(void *));
 
        if (!s->cpu_slab)
                return 0;