Re: [PATCH -mm v2 5/8] slub: make slab_free non-preemptable

Joonsoo Kim
2014-06-12T06:54:46Z

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@parallels.com>
> ---
> 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.