[PATCH] slab: removes local_irq_save()/local_irq_restore() pair

Proposed by and based on a patch from Eric Dumazet <dada1@cosmosbay.com>:
This patch removes unnecessary critical section in ksize() function, as
cli/sti are rather expensive on modern CPUS.

It additionally adds a docbook entry for ksize() and further simplifies the
code.

Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -3076,20 +3076,24 @@ ssize_t slabinfo_write(struct file *file, const char __user *buffer,
 }
 #endif
 
+/**
+ * ksize - get the actual amount of memory allocated for a given object
+ * @objp: Pointer to the object
+ *
+ * kmalloc may internally round up allocations and return more memory
+ * than requested. ksize() can be used to determine the actual amount of
+ * memory allocated. The caller may use this additional memory, even though
+ * a smaller amount of memory was initially specified with the kmalloc call.
+ * The caller must guarantee that objp points to a valid object previously
+ * allocated with either kmalloc() or kmem_cache_alloc(). The object
+ * must not be freed during the duration of the call.
+ */
 unsigned int ksize(const void *objp)
 {
-	kmem_cache_t *c;
-	unsigned long flags;
-	unsigned int size = 0;
-
-	if (likely(objp != NULL)) {
-		local_irq_save(flags);
-		c = GET_PAGE_CACHE(virt_to_page(objp));
-		size = kmem_cache_size(c);
-		local_irq_restore(flags);
-	}
+	if (unlikely(objp == NULL))
+		return 0;
 
-	return size;
+	return obj_reallen(GET_PAGE_CACHE(virt_to_page(objp)));
 }