[PATCH] mm: slab: Only move management objects off-slab for sizes larger than KMALLOC_MIN_SIZE

From: Catalin Marinas
Date: Thu Nov 05 2015 - 06:50:59 EST

Commit 8fc9cf420b36 ("slab: make more slab management structure off the
slab") enables off-slab management objects for sizes starting with
PAGE_SIZE >> 5. This means 128 bytes for a 4KB page configuration.
However, on systems with a KMALLOC_MIN_SIZE of 128 (arm64 in 4.4), such
optimisation does not make sense since the slab management allocation
would take 128 bytes anyway (even though freelist_size is 32) with the
additional overhead of another allocation.

This patch introduces an OFF_SLAB_MIN_SIZE macro which takes
KMALLOC_MIN_SIZE into account. It also solves a slab bug on arm64 where
the first kmalloc_cache to be initialised after slab_early_init = 0,
"kmalloc-128", fails to allocate off-slab management objects from the
same "kmalloc-128" cache.

Fixes: 8fc9cf420b36 ("slab: make more slab management structure off the slab")
Cc: <stable@xxxxxxxxxxxxxxx> # 3.15+
Reported-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
mm/slab.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 4fcc5dd8d5a6..419b649b395e 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -282,6 +282,7 @@ static void kmem_cache_node_init(struct kmem_cache_node *parent)

#define CFLGS_OFF_SLAB (0x80000000UL)
#define OFF_SLAB(x) ((x)->flags & CFLGS_OFF_SLAB)
+#define OFF_SLAB_MIN_SIZE (max_t(size_t, PAGE_SIZE >> 5, KMALLOC_MIN_SIZE + 1))

@@ -2212,7 +2213,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
* it too early on. Always use on-slab management when
* SLAB_NOLEAKTRACE to avoid recursive calls into kmemleak)
- if ((size >= (PAGE_SIZE >> 5)) && !slab_early_init &&
+ if (size >= OFF_SLAB_MIN_SIZE && !slab_early_init &&
* Size is large, assume best to place the slab management obj
@@ -2276,7 +2277,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
* This is a possibility for one of the kmalloc_{dma,}_caches.
* But since we go off slab only for object size greater than
- * PAGE_SIZE/8, and kmalloc_{dma,}_caches get created
+ * OFF_SLAB_MIN_SIZE, and kmalloc_{dma,}_caches get created
* in ascending order,this should not happen at all.
* But leave a BUG_ON for some lucky dude.
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