On Wed, 30 Sep 2015 13:44:19 +0200 Jesper Dangaard Brouer [off-list ref] wrote:

Make it possible to free a freelist with several objects by adjusting
API of slab_free() and __slab_free() to have head, tail and an objects
counter (cnt).

Tail being NULL indicate single object free of head object.  This
allow compiler inline constant propagation in slab_free() and
slab_free_freelist_hook() to avoid adding any overhead in case of
single object free.

This allows a freelist with several objects (all within the same
slab-page) to be free'ed using a single locked cmpxchg_double in
__slab_free() and with an unlocked cmpxchg_double in slab_free().

Object debugging on the free path is also extended to handle these
freelists.  When CONFIG_SLUB_DEBUG is enabled it will also detect if
objects don't belong to the same slab-page.

These changes are needed for the next patch to bulk free the detached
freelists it introduces and constructs.

Micro benchmarking showed no performance reduction due to this change,
when debugging is turned off (compiled with CONFIG_SLUB_DEBUG).

checkpatch says

WARNING: Avoid crashing the kernel - try using WARN_ON & recovery code rather than BUG() or BUG_ON()
#205: FILE: mm/slub.c:2888:
+       BUG_ON(!size);


Linus will get mad at you if he finds out, and we wouldn't want that.

--- a/mm/slub.c~slub-optimize-bulk-slowpath-free-by-detached-freelist-fix
+++ a/mm/slub.c

@@ -2885,7 +2885,8 @@ static int build_detached_freelist(struc
 /* Note that interrupts must be enabled when calling this function. */
 void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
 {
-	BUG_ON(!size);
+	if (WARN_ON(!size))
+		return;
 
 	do {
 		struct detached_freelist df;

_