On Fri, Jun 06, 2014 at 05:22:42PM +0400, Vladimir Davydov wrote: > Since per memcg cache destruction is scheduled when the last slab is > freed, to avoid use-after-free in kmem_cache_free we should either > rearrange code in kmem_cache_free so that it won't dereference the cache > ptr after freeing the object, or wait for all kmem_cache_free's to > complete before proceeding to cache destruction. > > The former approach isn't a good option from the future development > point of view, because every modifications to kmem_cache_free must be > done with great care then. Hence we should provide a method to wait for > all currently executing kmem_cache_free's to finish. > > This patch makes SLUB's implementation of kmem_cache_free > non-preemptable. As a result, synchronize_sched() will work as a barrier > against kmem_cache_free's in flight, so that issuing it before cache > destruction will protect us against the use-after-free. > > This won't affect performance of kmem_cache_free, because we already > disable preemption there, and this patch only moves preempt_enable to > the end of the function. Neither should it affect the system latency, > because kmem_cache_free is extremely short, even in its slow path. > > SLAB's version of kmem_cache_free already proceeds with irqs disabled, > so nothing to be done there. > > Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> > --- > mm/slub.c | 10 ++-------- > 1 file changed, 2 insertions(+), 8 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 35741592be8c..e46d6abe8a68 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2673,18 +2673,11 @@ static __always_inline void slab_free(struct kmem_cache *s, > > slab_free_hook(s, x); > > -redo: > - /* > - * Determine the currently cpus per cpu slab. > - * The cpu may change afterward. However that does not matter since > - * data is retrieved via this pointer. If we are on the same cpu > - * during the cmpxchg then the free will succedd. > - */ > preempt_disable(); Hello, Could you add some code comment why this preempt_disable/enable() is needed? We don't have any clue that kmemcg depends on these things on code, so someone cannot understand why it is here. If possible, please add similar code comment on slab_alloc in mm/slab.c. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>