David Miller wrote:
From: Christoph Lameter <[email protected]>
Date: Tue, 8 May 2007 18:32:35 -0700 (PDT)
That SLUB cannot do. And I do not believe you. SLOB must have some way to
distinguish the objects and their sizes since kfree does not include size
information. You can mix slabs of different size on the same page without
metadata. Magic?
So how does kfree then know how to free the object? There must be some way
where you get the metainformation. What is the point of your 8 byte
metadata that keeps getting inserted? That does not consume memory on a
page?
SLOB uses metadata, but that metadata seemingly only needs to be
uptodate in freed objects.
SLOB seems to look at the descriptor in the previous blob to figure
out how big the being-freed blob is. That's actually kind of clever
:-)
You know how big the being-freed blob is because the kmem cache structure
contains that. The free metadata is just needed for free area management.
BTW, we _really_ should be doing RCU properly in slob, because you
technically can't noop RCU on UP (even though the current users may be
safe...).
Patch attached to do that.
--
SUSE Labs, Novell Inc.
Index: linux-2.6/mm/slob.c
===================================================================
--- linux-2.6.orig/mm/slob.c 2007-04-12 14:35:11.000000000 +1000
+++ linux-2.6/mm/slob.c 2007-05-09 09:22:33.000000000 +1000
@@ -35,6 +35,7 @@
#include <linux/init.h>
#include <linux/module.h>
#include <linux/timer.h>
+#include <linux/rcupdate.h>
struct slob_block {
int units;
@@ -53,6 +54,11 @@
};
typedef struct bigblock bigblock_t;
+struct slob_rcu {
+ struct rcu_head rcu_head;
+ int size;
+};
+
static slob_t arena = { .next = &arena, .units = 1 };
static slob_t *slobfree = &arena;
static bigblock_t *bigblocks;
@@ -242,6 +248,7 @@
struct kmem_cache {
unsigned int size, align;
+ unsigned long flags;
const char *name;
void (*ctor)(void *, struct kmem_cache *, unsigned long);
void (*dtor)(void *, struct kmem_cache *, unsigned long);
@@ -259,6 +266,11 @@
if (c) {
c->name = name;
c->size = size;
+ if (flags & SLAB_DESTROY_BY_RCU) {
+ /* leave room for rcu header at the start of object */
+ c->size += sizeof(struct slob_rcu);
+ }
+ c->flags = flags;
c->ctor = ctor;
c->dtor = dtor;
/* ignore alignment unless it's forced */
@@ -281,11 +293,14 @@
{
void *b;
- if (c->size < PAGE_SIZE)
+ if (c->size < PAGE_SIZE) {
b = slob_alloc(c->size, flags, c->align);
- else
+ } else
b = (void *)__get_free_pages(flags, find_order(c->size));
+ if (unlikely(c->flags & SLAB_DESTROY_BY_RCU))
+ b += sizeof(struct slob_rcu);
+
if (c->ctor)
c->ctor(b, c, SLAB_CTOR_CONSTRUCTOR);
@@ -303,15 +318,34 @@
}
EXPORT_SYMBOL(kmem_cache_zalloc);
+static void __kmem_cache_free(void *b, int size)
+{
+ if (size < PAGE_SIZE)
+ slob_free(b, size);
+ else
+ free_pages((unsigned long)b, find_order(size));
+}
+
+static void kmem_rcu_free(struct rcu_head *head)
+{
+ struct slob_rcu *slob_rcu = (struct slob_rcu *)head;
+
+ __kmem_cache_free(head, slob_rcu->size);
+}
+
void kmem_cache_free(struct kmem_cache *c, void *b)
{
if (c->dtor)
c->dtor(b, c, 0);
- if (c->size < PAGE_SIZE)
- slob_free(b, c->size);
- else
- free_pages((unsigned long)b, find_order(c->size));
+ if (unlikely(c->flags & SLAB_DESTROY_BY_RCU)) {
+ struct slob_rcu *slob_rcu;
+ b -= sizeof(struct slob_rcu);
+ slob_rcu = b;
+ slob_rcu->size = c->size;
+ call_rcu(&slob_rcu->rcu_head, kmem_rcu_free);
+ } else
+ __kmem_cache_free(b, c->size);
}
EXPORT_SYMBOL(kmem_cache_free);
Index: linux-2.6/init/Kconfig
===================================================================
--- linux-2.6.orig/init/Kconfig 2007-04-12 14:35:11.000000000 +1000
+++ linux-2.6/init/Kconfig 2007-05-09 09:11:14.000000000 +1000
@@ -476,7 +476,7 @@
config SLAB
default y
- bool "Use full SLAB allocator" if (EMBEDDED && !SMP && !SPARSEMEM)
+ bool "Use full SLAB allocator" if (EMBEDDED && !SPARSEMEM)
help
Disabling this replaces the advanced SLAB allocator and
kmalloc support with the drastically simpler SLOB allocator.
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