[PATCH] implement kmem_cache_size()

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.  Here's the patch:

include/linux/slab.h

@@ -57,6 +57,7 @@ extern int kmem_cache_destroy(kmem_cache_t *);
 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 *);

kernel/ksyms.c

@@ -105,6 +105,7 @@ EXPORT_SYMBOL(kmem_cache_destroy);
 EXPORT_SYMBOL(kmem_cache_shrink);
 EXPORT_SYMBOL(kmem_cache_alloc);
 EXPORT_SYMBOL(kmem_cache_free);
+EXPORT_SYMBOL(kmem_cache_size);
 EXPORT_SYMBOL(kmalloc);
 EXPORT_SYMBOL(kfree);
 EXPORT_SYMBOL(vfree);

mm/slab.c

@@ -1647,6 +1647,15 @@ void kfree (const void *objp)
 	local_irq_restore(flags);
 }
 
+unsigned int kmem_cache_size(kmem_cache_t *cachep)
+{
+#if DEBUG
+	if (cachep->flags & SLAB_RED_ZONE)
+		return (cachep->objsize - 2*BYTES_PER_WORD);
+#endif
+	return cachep->objsize;
+}
+
 kmem_cache_t * kmem_find_general_cachep (size_t size, int gfpflags)
 {
 	cache_sizes_t *csizep = cache_sizes;