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

[Christoph Lameter]

On Thu, 17 Jan 2013, Steven Rostedt wrote:

> > --- 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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