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Subject[TRIVIAL] implement kmem_cache_size
From:  Christoph Hellwig <>

Currently there is no way to find out the effective object size of a slab
cache. XFS has lots of IRIX-derived code that want to do zalloc() style
allocations on zones (which are implemented as slab caches in XFS/Linux)
and thus needs to know about it. There are three ways do implement it:

a) implement kmem_cache_zalloc
b) make the xfs zone a struct of kmem_cache_t and a size variable
c) implement kmem_cache_size

The current XFS tree does a) but I absolutely don't like it as encourages
people to use kmem_cache_zalloc for new code instead of thinking about
how to utilize slab object reuse. b) would be easy, but I guess
kmem_cache_size is usefull enough to get into the kernel.

Trivial patch to implement kmem_cache_size (doesn't change any existing
code) appended:

--- trivial-2.5.28/include/linux/slab.h.orig Fri Jul 26 16:37:00 2002
+++ trivial-2.5.28/include/linux/slab.h Fri Jul 26 16:37:00 2002
@@ -57,6 +57,7 @@
extern int kmem_cache_shrink(kmem_cache_t *);
extern void *kmem_cache_alloc(kmem_cache_t *, int);
extern void kmem_cache_free(kmem_cache_t *, void *);
+extern unsigned int kmem_cache_size(kmem_cache_t *);

extern void *kmalloc(size_t, int);
extern void kfree(const void *);
--- trivial-2.5.28/kernel/ksyms.c.orig Fri Jul 26 16:37:00 2002
+++ trivial-2.5.28/kernel/ksyms.c Fri Jul 26 16:37:00 2002
@@ -105,6 +105,7 @@
--- trivial-2.5.28/mm/slab.c.orig Fri Jul 26 16:37:00 2002
+++ trivial-2.5.28/mm/slab.c Fri Jul 26 16:37:00 2002
@@ -1647,6 +1647,15 @@

+unsigned int kmem_cache_size(kmem_cache_t *cachep)
+#if DEBUG
+ if (cachep->flags & SLAB_RED_ZONE)
+ return (cachep->objsize - 2*BYTES_PER_WORD);
+ return cachep->objsize;
kmem_cache_t * kmem_find_general_cachep (size_t size, int gfpflags)
cache_sizes_t *csizep = cache_sizes;
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