Re: [PATCH] mm,slab,memcg: call memcg kmem put cache with same condition as get

[Shakeel Butt]

On Tue, Jan 8, 2019 at 8:01 PM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> There is an imbalance between when slab_pre_alloc_hook calls
> memcg_kmem_get_cache and when slab_post_alloc_hook calls
> memcg_kmem_put_cache.
>

Can you explain how there is an imbalance? If the returned kmem cache
from memcg_kmem_get_cache() is the memcg kmem cache then the refcnt of
memcg is elevated and the memcg_kmem_put_cache() will correctly
decrement the refcnt of the memcg.

> This can cause a memcg kmem cache to be destroyed right as
> an object from that cache is being allocated, which is probably
> not good. It could lead to things like a memcg allocating new
> kmalloc slabs instead of using freed space in old ones, maybe
> memory leaks, and maybe oopses as a memcg kmalloc slab is getting
> destroyed on one CPU while another CPU is trying to do an allocation
> from that same memcg.
>
> The obvious fix would be to use the same condition for calling
> memcg_kmem_put_cache that we also use to decide whether to call
> memcg_kmem_get_cache.
>
> I am not sure how long this bug has been around, since the last
> changeset to touch that code - 452647784b2f ("mm: memcontrol: cleanup
>  kmem charge functions") - merely moved the bug from one location to
> another. I am still tagging that changeset, because the fix should
> automatically apply that far back.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> Fixes: 452647784b2f ("mm: memcontrol: cleanup kmem charge functions")
> Cc: kernel-team@xxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  mm/slab.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 4190c24ef0e9..ab3d95bef8a0 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -444,7 +444,8 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
>                 p[i] = kasan_slab_alloc(s, object, flags);
>         }
>
> -       if (memcg_kmem_enabled())
> +       if (memcg_kmem_enabled() &&
> +           ((flags & __GFP_ACCOUNT) || (s->flags & SLAB_ACCOUNT)))

I don't think these extra checks are needed. They are safe but not needed.

>                 memcg_kmem_put_cache(s);
>  }
>
> --
> 2.17.1
>

thanks,
Shakeel