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SubjectRe: [PATCH v3 04/19] mm: slub: implement SLUB version of obj_to_index()
On Wed, May 20, 2020 at 03:51:45PM +0200, Vlastimil Babka wrote:
> On 4/22/20 10:46 PM, Roman Gushchin wrote:
> > This commit implements SLUB version of the obj_to_index() function,
> > which will be required to calculate the offset of obj_cgroup in the
> > obj_cgroups vector to store/obtain the objcg ownership data.
> >
> > To make it faster, let's repeat the SLAB's trick introduced by
> > commit 6a2d7a955d8d ("[PATCH] SLAB: use a multiply instead of a
> > divide in obj_to_index()") and avoid an expensive division.
> >
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Acked-by: Christoph Lameter <cl@linux.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> There's already a slab_index() doing the same without the trick, with only
> SLUB_DEBUG callers. Maybe just improve it and perhaps rename? (obj_to_index()
> seems more descriptive). The difference is that it takes the result of
> page_addr() instead of doing that, as it's being called in a loop on objects
> from a single page, so you'd have to perhaps split to obj_to_index(page) and
> __obj_to_index(addr) or something.

Good point! How about this one?

--

From beeaecdac85c3a395dcfb99944dc8c858b541cbf Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Mon, 29 Jul 2019 18:18:42 -0700
Subject: [PATCH v3.2 04/19] mm: slub: implement SLUB version of obj_to_index()

This commit implements SLUB version of the obj_to_index() function,
which will be required to calculate the offset of obj_cgroup in the
obj_cgroups vector to store/obtain the objcg ownership data.

To make it faster, let's repeat the SLAB's trick introduced by
commit 6a2d7a955d8d ("[PATCH] SLAB: use a multiply instead of a
divide in obj_to_index()") and avoid an expensive division.

Vlastimil Babka noticed, that SLUB does have already a similar
function called slab_index(), which is defined only if SLUB_DEBUG
is enabled. The function does a similar math, but with a division,
and it also takes a page address instead of a page pointer.

Let's remove slab_index() and replace it with the new helper
__obj_to_index(), which takes a page address. obj_to_index()
will be a simple wrapper taking a page pointer and passing
page_address(page) into __obj_to_index().

Signed-off-by: Roman Gushchin <guro@fb.com>
---
include/linux/slub_def.h | 16 ++++++++++++++++
mm/slub.c | 15 +++++----------
2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index d2153789bd9f..30e91c83d401 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -8,6 +8,7 @@
* (C) 2007 SGI, Christoph Lameter
*/
#include <linux/kobject.h>
+#include <linux/reciprocal_div.h>

enum stat_item {
ALLOC_FASTPATH, /* Allocation from cpu slab */
@@ -86,6 +87,7 @@ struct kmem_cache {
unsigned long min_partial;
unsigned int size; /* The size of an object including metadata */
unsigned int object_size;/* The size of an object without metadata */
+ struct reciprocal_value reciprocal_size;
unsigned int offset; /* Free pointer offset */
#ifdef CONFIG_SLUB_CPU_PARTIAL
/* Number of per cpu partial objects to keep around */
@@ -182,4 +184,18 @@ static inline void *nearest_obj(struct kmem_cache *cache, struct page *page,
return result;
}

+/* Determine object index from a given position */
+static inline unsigned int __obj_to_index(const struct kmem_cache *cache,
+ void *addr, void *obj)
+{
+ return reciprocal_divide(kasan_reset_tag(obj) - addr,
+ cache->reciprocal_size);
+}
+
+static inline unsigned int obj_to_index(const struct kmem_cache *cache,
+ const struct page *page, void *obj)
+{
+ return __obj_to_index(cache, page_address(page), obj);
+}
+
#endif /* _LINUX_SLUB_DEF_H */
diff --git a/mm/slub.c b/mm/slub.c
index 2df4d4a420d1..d605d18b3c1b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -313,12 +313,6 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
__p < (__addr) + (__objects) * (__s)->size; \
__p += (__s)->size)

-/* Determine object index from a given position */
-static inline unsigned int slab_index(void *p, struct kmem_cache *s, void *addr)
-{
- return (kasan_reset_tag(p) - addr) / s->size;
-}
-
static inline unsigned int order_objects(unsigned int order, unsigned int size)
{
return ((unsigned int)PAGE_SIZE << order) / size;
@@ -461,7 +455,7 @@ static unsigned long *get_map(struct kmem_cache *s, struct page *page)
bitmap_zero(object_map, page->objects);

for (p = page->freelist; p; p = get_freepointer(s, p))
- set_bit(slab_index(p, s, addr), object_map);
+ set_bit(__obj_to_index(s, addr, p), object_map);

return object_map;
}
@@ -3682,6 +3676,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
*/
size = ALIGN(size, s->align);
s->size = size;
+ s->reciprocal_size = reciprocal_value(size);
if (forced_order >= 0)
order = forced_order;
else
@@ -3788,7 +3783,7 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
map = get_map(s, page);
for_each_object(p, s, addr, page->objects) {

- if (!test_bit(slab_index(p, s, addr), map)) {
+ if (!test_bit(__obj_to_index(s, addr, p), map)) {
pr_err("INFO: Object 0x%p @offset=%tu\n", p, p - addr);
print_tracking(s, p);
}
@@ -4513,7 +4508,7 @@ static void validate_slab(struct kmem_cache *s, struct page *page)
/* Now we know that a valid freelist exists */
map = get_map(s, page);
for_each_object(p, s, addr, page->objects) {
- u8 val = test_bit(slab_index(p, s, addr), map) ?
+ u8 val = test_bit(__obj_to_index(s, addr, p), map) ?
SLUB_RED_INACTIVE : SLUB_RED_ACTIVE;

if (!check_object(s, page, p, val))
@@ -4704,7 +4699,7 @@ static void process_slab(struct loc_track *t, struct kmem_cache *s,

map = get_map(s, page);
for_each_object(p, s, addr, page->objects)
- if (!test_bit(slab_index(p, s, addr), map))
+ if (!test_bit(__obj_to_index(s, addr, p), map))
add_location(t, s, get_track(s, p, alloc));
put_map(map);
}
--
2.25.4
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