From: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Subject: slab: make kmalloc_index() return "unsigned int"
kmalloc_index() return index into an array of kmalloc kmem caches,
therefore should be unsigned.
Space savings with SLUB on trimmed down .config:
add/remove: 0/1 grow/shrink: 6/56 up/down: 85/-557 (-472)
Function old new delta
calculate_sizes 924 983 +59
on_freelist 589 604 +15
init_cache_random_seq 122 127 +5
ext4_mb_init 1206 1210 +4
slab_pad_check.part 270 271 +1
cpu_partial_store 112 113 +1
usersize_show 28 27 -1
...
new_slab 1871 1837 -34
slab_order 204 - -204
This patch start a series of converting SLUB (mostly) to "unsigned int".
1) Most integers in the code are in fact unsigned entities: array indexes,
lengths, buffer sizes, allocation orders. It is therefore better to use
unsigned variables
2) Some integers in the code are either "size_t" or "unsigned long"
for no reason.
size_t usually comes from people trying to maintain type correctness
and figuring out that "sizeof" operator returns size_t or memset/memcpy
takes size_t so should everything passed to it.
However the number of 4GB+ objects in the kernel is very small.
Most, if not all, dynamically allocated objects with kmalloc() or
kmem_cache_create() aren't actually big. Maintaining wide types
doesn't do anything.
64-bit ops are bigger than 32-bit on our beloved x86_64,
so try to not use 64-bit where it isn't necessary
(read: everywhere where integers are integers not pointers)
3) in case of SLAB allocators, there are additional limitations
*) page->inuse, page->objects are only 16-/15-bit,
*) cache size was always 32-bit
*) slab orders are small, order 20 is needed to go 64-bit on x86_64
(PAGE_SIZE << order)
Basically everything is 32-bit except kmalloc(1ULL<<32) which gets shortcut
through page allocator.
Christoph said:
: That changes with large base page size on power and ARM64 f.e. but then
: we do not want to encourage larger allocations through slab anyways.
Link: http://lkml.kernel.org/r/20180305200730.15812-2-adobriyan@xxxxxxxxx
Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/slab.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff -puN include/linux/slab.h~slab-make-kmalloc_index-return-unsigned-int include/linux/slab.h
--- a/include/linux/slab.h~slab-make-kmalloc_index-return-unsigned-int
+++ a/include/linux/slab.h
@@ -308,7 +308,7 @@ extern struct kmem_cache *kmalloc_dma_ca
* 2 = 129 .. 192 bytes
* n = 2^(n-1)+1 .. 2^n
*/
-static __always_inline int kmalloc_index(size_t size)
+static __always_inline unsigned int kmalloc_index(size_t size)
{
if (!size)
return 0;
@@ -504,7 +504,7 @@ static __always_inline void *kmalloc(siz
return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
if (!(flags & GFP_DMA)) {
- int index = kmalloc_index(size);
+ unsigned int index = kmalloc_index(size);
if (!index)
return ZERO_SIZE_PTR;
@@ -542,7 +542,7 @@ static __always_inline void *kmalloc_nod
#ifndef CONFIG_SLOB
if (__builtin_constant_p(size) &&
size <= KMALLOC_MAX_CACHE_SIZE && !(flags & GFP_DMA)) {
- int i = kmalloc_index(size);
+ unsigned int i = kmalloc_index(size);
if (!i)
return ZERO_SIZE_PTR;
_
--
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