[PATCH] implement kmem_cache_size

[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.  Here's the patch:

# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or higher.
# This patch includes the following deltas:
#	           ChangeSet	1.641   -> 1.642  
#	      kernel/ksyms.c	1.109   -> 1.110  
#	include/linux/slab.h	1.11    -> 1.12   
#	           mm/slab.c	1.21    -> 1.22   
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 02/07/15	[e-mail]	1.642
# Add a new function, kmem_cache_size to the slab allocator implementation.
# 
# It is used to find out the effective object size of a slab cache,
# which is needed to e.g. implement a zalloc() lookalike in XFS.
# --------------------------------------------
#
diff -Nru a/include/linux/slab.h b/include/linux/slab.h
--- a/include/linux/slab.h	Mon Jul 15 16:39:25 2002
+++ b/include/linux/slab.h	Mon Jul 15 16:39:25 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 *);
diff -Nru a/kernel/ksyms.c b/kernel/ksyms.c
--- a/kernel/ksyms.c	Mon Jul 15 16:39:25 2002
+++ b/kernel/ksyms.c	Mon Jul 15 16:39:25 2002
@@ -105,6 +105,7 @@
 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);
diff -Nru a/mm/slab.c b/mm/slab.c
--- a/mm/slab.c	Mon Jul 15 16:39:25 2002
+++ b/mm/slab.c	Mon Jul 15 16:39:25 2002
@@ -1637,6 +1637,15 @@
 	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;