[PATCH V2 1/3] slub: extend slowpath __slab_free() to handle bulk free

[Jesper Dangaard Brouer <brouer@xxxxxxxxxx>]

Make it possible to free a freelist with several objects by extending
__slab_free() with two arguments: a freelist_head pointer and objects
counter (cnt).  If freelist_head pointer is set, then the object must
be the freelist tail pointer.

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

Micro benchmarking showed no performance reduction due to this change.

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

---
V2: Per request of Christoph Lameter
 * Made it more clear that freelist objs must exist within same page

 mm/slub.c |   16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index c9305f525004..10b57a3bb895 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2573,9 +2573,14 @@ EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
  * So we still attempt to reduce cache line usage. Just take the slab
  * lock and free the item. If there is no additional partial page
  * handling required then we can return immediately.
+ *
+ * Bulk free of a freelist with several objects (all pointing to the
+ * same page) possible by specifying freelist_head ptr and object as
+ * tail ptr, plus objects count (cnt).
  */
 static void __slab_free(struct kmem_cache *s, struct page *page,
-			void *x, unsigned long addr)
+			void *x, unsigned long addr,
+			void *freelist_head, int cnt)
 {
 	void *prior;
 	void **object = (void *)x;
@@ -2584,6 +2589,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 	unsigned long counters;
 	struct kmem_cache_node *n = NULL;
 	unsigned long uninitialized_var(flags);
+	void *new_freelist = (!freelist_head) ? object : freelist_head;
 
 	stat(s, FREE_SLOWPATH);
 
@@ -2601,7 +2607,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 		set_freepointer(s, object, prior);
 		new.counters = counters;
 		was_frozen = new.frozen;
-		new.inuse--;
+		new.inuse -= cnt;
 		if ((!new.inuse || !prior) && !was_frozen) {
 
 			if (kmem_cache_has_cpu_partial(s) && !prior) {
@@ -2632,7 +2638,7 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 
 	} while (!cmpxchg_double_slab(s, page,
 		prior, counters,
-		object, new.counters,
+		new_freelist, new.counters,
 		"__slab_free"));
 
 	if (likely(!n)) {
@@ -2736,7 +2742,7 @@ redo:
 		}
 		stat(s, FREE_FASTPATH);
 	} else
-		__slab_free(s, page, x, addr);
+		__slab_free(s, page, x, addr, NULL, 1);
 
 }
 
@@ -2780,7 +2786,7 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
 			c->tid = next_tid(c->tid);
 			local_irq_enable();
 			/* Slowpath: overhead locked cmpxchg_double_slab */
-			__slab_free(s, page, object, _RET_IP_);
+			__slab_free(s, page, object, _RET_IP_, NULL, 1);
 			local_irq_disable();
 			c = this_cpu_ptr(s->cpu_slab);
 		}

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