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

[Eric Dumazet]

On Thu, 2013-01-17 at 21:28 +0000, Christoph Lameter wrote:
> 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;
>  	}
> 

Without appropriate barriers, this change is a shoot in the dark.





--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/