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Subject[PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE
This patch introduces a flag SLAB_MINIMIZE_WASTE for slab and slub. This
flag causes allocation of larger slab caches in order to minimize wasted
space.

This is needed because we want to use dm-bufio for deduplication index and
there are existing installations with non-power-of-two block sizes (such
as 640KB). The performance of the whole solution depends on efficient
memory use, so we must waste as little memory as possible.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
drivers/md/dm-bufio.c | 2 +-
include/linux/slab.h | 7 +++++++
mm/slab.c | 4 ++--
mm/slab.h | 7 ++++---
mm/slab_common.c | 2 +-
mm/slub.c | 25 ++++++++++++++++++++-----
6 files changed, 35 insertions(+), 12 deletions(-)

Index: linux-2.6/include/linux/slab.h
===================================================================
--- linux-2.6.orig/include/linux/slab.h 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/include/linux/slab.h 2018-04-16 21:10:45.000000000 +0200
@@ -108,6 +108,13 @@
#define SLAB_KASAN 0
#endif

+/*
+ * Use higer order allocations to minimize wasted space.
+ * Note: the allocation is unreliable if this flag is used, the caller
+ * must handle allocation failures gracefully.
+ */
+#define SLAB_MINIMIZE_WASTE ((slab_flags_t __force)0x10000000U)
+
/* The following flags affect the page allocator grouping pages by mobility */
/* Objects are reclaimable */
#define SLAB_RECLAIM_ACCOUNT ((slab_flags_t __force)0x00020000U)
Index: linux-2.6/mm/slab_common.c
===================================================================
--- linux-2.6.orig/mm/slab_common.c 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/mm/slab_common.c 2018-04-16 21:10:45.000000000 +0200
@@ -53,7 +53,7 @@ static DECLARE_WORK(slab_caches_to_rcu_d
SLAB_FAILSLAB | SLAB_KASAN)

#define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
- SLAB_ACCOUNT)
+ SLAB_ACCOUNT | SLAB_MINIMIZE_WASTE)

/*
* Merge control. If this is set then no merging of slab caches will occur.
Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/mm/slub.c 2018-04-16 21:12:41.000000000 +0200
@@ -3249,7 +3249,7 @@ static inline unsigned int slab_order(un
return order;
}

-static inline int calculate_order(unsigned int size, unsigned int reserved)
+static inline int calculate_order(unsigned int size, unsigned int reserved, slab_flags_t flags)
{
unsigned int order;
unsigned int min_objects;
@@ -3277,7 +3277,7 @@ static inline int calculate_order(unsign
order = slab_order(size, min_objects,
slub_max_order, fraction, reserved);
if (order <= slub_max_order)
- return order;
+ goto ret_order;
fraction /= 2;
}
min_objects--;
@@ -3289,15 +3289,30 @@ static inline int calculate_order(unsign
*/
order = slab_order(size, 1, slub_max_order, 1, reserved);
if (order <= slub_max_order)
- return order;
+ goto ret_order;

/*
* Doh this slab cannot be placed using slub_max_order.
*/
order = slab_order(size, 1, MAX_ORDER, 1, reserved);
if (order < MAX_ORDER)
- return order;
+ goto ret_order;
return -ENOSYS;
+
+ret_order:
+ if (flags & SLAB_MINIMIZE_WASTE) {
+ /* Increase the order if it decreases waste */
+ int test_order;
+ for (test_order = order + 1; test_order < MAX_ORDER; test_order++) {
+ unsigned long order_objects = ((PAGE_SIZE << order) - reserved) / size;
+ unsigned long test_order_objects = ((PAGE_SIZE << test_order) - reserved) / size;
+ if (test_order_objects >= min(32, MAX_OBJS_PER_PAGE))
+ break;
+ if (test_order_objects > order_objects << (test_order - order))
+ order = test_order;
+ }
+ }
+ return order;
}

static void
@@ -3562,7 +3577,7 @@ static int calculate_sizes(struct kmem_c
if (forced_order >= 0)
order = forced_order;
else
- order = calculate_order(size, s->reserved);
+ order = calculate_order(size, s->reserved, flags);

if ((int)order < 0)
return 0;
Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/drivers/md/dm-bufio.c 2018-04-16 21:11:23.000000000 +0200
@@ -1683,7 +1683,7 @@ struct dm_bufio_client *dm_bufio_client_
(block_size < PAGE_SIZE || !is_power_of_2(block_size))) {
snprintf(slab_name, sizeof slab_name, "dm_bufio_cache-%u", c->block_size);
c->slab_cache = kmem_cache_create(slab_name, c->block_size, ARCH_KMALLOC_MINALIGN,
- SLAB_RECLAIM_ACCOUNT, NULL);
+ SLAB_RECLAIM_ACCOUNT | SLAB_MINIMIZE_WASTE, NULL);
if (!c->slab_cache) {
r = -ENOMEM;
goto bad;
Index: linux-2.6/mm/slab.h
===================================================================
--- linux-2.6.orig/mm/slab.h 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/mm/slab.h 2018-04-16 21:10:45.000000000 +0200
@@ -142,10 +142,10 @@ static inline slab_flags_t kmem_cache_fl
#if defined(CONFIG_SLAB)
#define SLAB_CACHE_FLAGS (SLAB_MEM_SPREAD | SLAB_NOLEAKTRACE | \
SLAB_RECLAIM_ACCOUNT | SLAB_TEMPORARY | \
- SLAB_ACCOUNT)
+ SLAB_ACCOUNT | SLAB_MINIMIZE_WASTE)
#elif defined(CONFIG_SLUB)
#define SLAB_CACHE_FLAGS (SLAB_NOLEAKTRACE | SLAB_RECLAIM_ACCOUNT | \
- SLAB_TEMPORARY | SLAB_ACCOUNT)
+ SLAB_TEMPORARY | SLAB_ACCOUNT | SLAB_MINIMIZE_WASTE)
#else
#define SLAB_CACHE_FLAGS (0)
#endif
@@ -164,7 +164,8 @@ static inline slab_flags_t kmem_cache_fl
SLAB_NOLEAKTRACE | \
SLAB_RECLAIM_ACCOUNT | \
SLAB_TEMPORARY | \
- SLAB_ACCOUNT)
+ SLAB_ACCOUNT | \
+ SLAB_MINIMIZE_WASTE)

bool __kmem_cache_empty(struct kmem_cache *);
int __kmem_cache_shutdown(struct kmem_cache *);
Index: linux-2.6/mm/slab.c
===================================================================
--- linux-2.6.orig/mm/slab.c 2018-04-16 21:10:45.000000000 +0200
+++ linux-2.6/mm/slab.c 2018-04-16 21:10:45.000000000 +0200
@@ -1790,14 +1790,14 @@ static size_t calculate_slab_order(struc
* as GFP_NOFS and we really don't want to have to be allocating
* higher-order pages when we are unable to shrink dcache.
*/
- if (flags & SLAB_RECLAIM_ACCOUNT)
+ if (flags & SLAB_RECLAIM_ACCOUNT && !(flags & SLAB_MINIMIZE_WASTE))
break;

/*
* Large number of objects is good, but very large slabs are
* currently bad for the gfp()s.
*/
- if (gfporder >= slab_max_order)
+ if (gfporder >= slab_max_order && !(flags & SLAB_MINIMIZE_WASTE))
break;

/*
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