If an architecture supports CMPXCHG_LOCAL this shouldn't result automatically
in larger struct pages if the SLUB allocator is used. Instead introduce a new
config option "HAVE_ALIGNED_STRUCT_PAGE" which can be selected if a double
word aligned struct page is required.
Also update x86 Kconfig so that it should work as before.
Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
---
arch/Kconfig | 8 ++++++++
arch/x86/Kconfig | 1 +
include/linux/mm_types.h | 9 ++++-----
mm/slub.c | 6 +++---
4 files changed, 16 insertions(+), 8 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index 4b0669c..4b4a140 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -181,4 +181,12 @@ config HAVE_RCU_TABLE_FREE
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
+config HAVE_ALIGNED_STRUCT_PAGE
+ bool
+ help
+ This makes sure that struct pages are double word aligned and that
+ e.g. the SLUB allocator can perform double word atomic operations
+ on a struct page for better performance. However selecting this
+ might increase the size of a struct page by a word.
+
source "kernel/gcov/Kconfig"
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cb9a104..5115ce4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -75,6 +75,7 @@ config X86
select HAVE_BPF_JIT if (X86_64 && NET)
select CLKEVT_I8253
select ARCH_HAVE_NMI_SAFE_CMPXCHG
+ select HAVE_ALIGNED_STRUCT_PAGE if SLUB && !M386
config INSTRUCTION_DECODER
def_bool (KPROBES || PERF_EVENTS)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5b42f1b..3cc3062 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -151,12 +151,11 @@ struct page {
#endif
}
/*
- * If another subsystem starts using the double word pairing for atomic
- * operations on struct page then it must change the #if to ensure
- * proper alignment of the page struct.
+ * The struct page can be forced to be double word aligned so that atomic ops
+ * on double words work. The SLUB allocator can make use of such a feature.
*/
-#if defined(CONFIG_SLUB) && defined(CONFIG_CMPXCHG_LOCAL)
- __attribute__((__aligned__(2*sizeof(unsigned long))))
+#ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
+ __aligned(2 * sizeof(unsigned long))
#endif
;
diff --git a/mm/slub.c b/mm/slub.c
index 7d2a996..7669b4c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -366,7 +366,7 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct page *page
const char *n)
{
VM_BUG_ON(!irqs_disabled());
-#ifdef CONFIG_CMPXCHG_DOUBLE
+#if defined(CONFIG_CMPXCHG_DOUBLE) && defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
if (s->flags & __CMPXCHG_DOUBLE) {
if (cmpxchg_double(&page->freelist,
freelist_old, counters_old,
@@ -400,7 +400,7 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
void *freelist_new, unsigned long counters_new,
const char *n)
{
-#ifdef CONFIG_CMPXCHG_DOUBLE
+#if defined(CONFIG_CMPXCHG_DOUBLE) && defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
if (s->flags & __CMPXCHG_DOUBLE) {
if (cmpxchg_double(&page->freelist,
freelist_old, counters_old,
@@ -2990,7 +2990,7 @@ static int kmem_cache_open(struct kmem_cache *s,
}
}
-#ifdef CONFIG_CMPXCHG_DOUBLE
+#if defined(CONFIG_CMPXCHG_DOUBLE) && defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
if (system_has_cmpxchg_double() && (s->flags & SLAB_DEBUG_FLAGS) == 0)
/* Enable fast mode */
s->flags |= __CMPXCHG_DOUBLE;
--
1.7.7.1
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