Re: [RFC][PATCH] slub: Check for page NULL before doing the node_match check

On Thu, 17 Jan 2013, Steven Rostedt wrote:

quoted

--- a/mm/slub.c
+++ b/mm/slub.c

@@ -2399,7 +2399,7 @@ redo:

 	object = c->freelist;
 	page = c->page;
-	if (unlikely(!object || !node_match(page, node)))
+	if (unlikely(!object || !page || !node_match(page, node)))

I'm still trying to see if c->freelist != NULL and c->page == NULL isn't
a bug. The cmpxchg_doubles are a little confusing. If it's not expected
that page is NULL but freelist isn't than we need to figure out why it
happened.

hmmm.. We may want to change the sequence of updates to c->page and
c->freelist. Update c->freelist to be NULL first so that we always enter
the slow path for these cases where we can do more expensive
synchronization.

Index: linux/mm/slub.c
===================================================================

--- linux.orig/mm/slub.c	2013-01-15 10:42:08.490183607 -0600
+++ linux/mm/slub.c	2013-01-17 15:27:48.973051155 -0600

@@ -1993,8 +1993,8 @@ static inline void flush_slab(struct kme
 	deactivate_slab(s, c->page, c->freelist);

 	c->tid = next_tid(c->tid);
-	c->page = NULL;
 	c->freelist = NULL;
+	c->page = NULL;
 }

 /*

@@ -2227,8 +2227,8 @@ redo:
 	if (unlikely(!node_match(page, node))) {
 		stat(s, ALLOC_NODE_MISMATCH);
 		deactivate_slab(s, page, c->freelist);
-		c->page = NULL;
 		c->freelist = NULL;
+		c->page = NULL;
 		goto new_slab;
 	}

@@ -2239,8 +2239,8 @@ redo:
 	 */
 	if (unlikely(!pfmemalloc_match(page, gfpflags))) {
 		deactivate_slab(s, page, c->freelist);
-		c->page = NULL;
 		c->freelist = NULL;
+		c->page = NULL;
 		goto new_slab;
 	}


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