On Wed, Jul 15, 2020 at 01:32:00PM +0200, Vlastimil Babka wrote:
> On 7/7/20 8:27 AM, Muchun Song wrote:
> > If the kmem_cache refcount is greater than one, we should not
> > mark the root kmem_cache as dying. If we mark the root kmem_cache
> > dying incorrectly, the non-root kmem_cache can never be destroyed.
> > It resulted in memory leak when memcg was destroyed. We can use the
> > following steps to reproduce.
> >
> > 1) Use kmem_cache_create() to create a new kmem_cache named A.
> > 2) Coincidentally, the kmem_cache A is an alias for kmem_cache B,
> > so the refcount of B is just increased.
> > 3) Use kmem_cache_destroy() to destroy the kmem_cache A, just
> > decrease the B's refcount but mark the B as dying.
> > 4) Create a new memory cgroup and alloc memory from the kmem_cache
> > A. It leads to create a non-root kmem_cache for allocating.
> > 5) When destroy the memory cgroup created in the step 4), the
> > non-root kmem_cache can never be destroyed.
Hello, Muchun!
If the scenario above is accurate, it means that somebody is allocating
from the kmem_cache A (or it's memcg counterparts, doesn't matter) after
calling kmem_cache_destroy()? If so, it's an API violation, and the following
memory leak is a non-issue on the slab side. No one should allocate memory
after calling kmem_cache_destroy(). It has to be called after all outstanding
allocations are freed, and it should be literally the last operation
with the kmem_cache.
Kmem_cache aliasing/sharing, as well as memcg accounting implementation are
implementation details and should not affect the picture.
I wonder, did you see the problem in the wild? How does it look like?
Which kmem_cache is involved? Etc.
BTW, Vlastimil is absolutely right about stable backports and rework planned
for 5.9, but let's figure out the problem first.
Thank you!
> >
> > If we repeat steps 4) and 5), this will cause a lot of memory leak.
> > So only when refcount reach zero, we mark the root kmem_cache as dying.
> >
> > Fixes: 92ee383f6daa ("mm: fix race between kmem_cache destroy, create and deactivate")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> CC Roman, who worked in this area recently.
>
> Also why is this marked "[PATCH v5.4.y, v4.19.y]"? Has it been fixed otherwise
> in 5.5+ ?
>
> > ---
> > mm/slab_common.c | 43 +++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 41 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 8c1ffbf7de45..83ee6211aec7 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -258,6 +258,11 @@ static void memcg_unlink_cache(struct kmem_cache *s)
> > list_del(&s->memcg_params.kmem_caches_node);
> > }
> > }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > + return is_root_cache(s) && s->memcg_params.dying;
> > +}
> > #else
> > static inline int init_memcg_params(struct kmem_cache *s,
> > struct kmem_cache *root_cache)
> > @@ -272,6 +277,11 @@ static inline void destroy_memcg_params(struct kmem_cache *s)
> > static inline void memcg_unlink_cache(struct kmem_cache *s)
> > {
> > }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > + return false;
> > +}
> > #endif /* CONFIG_MEMCG_KMEM */
> >
> > /*
> > @@ -326,6 +336,13 @@ int slab_unmergeable(struct kmem_cache *s)
> > if (s->refcount < 0)
> > return 1;
> >
> > + /*
> > + * If the kmem_cache is dying. We should also skip this
> > + * kmem_cache.
> > + */
> > + if (memcg_kmem_cache_dying(s))
> > + return 1;
> > +
> > return 0;
> > }
> >
> > @@ -944,8 +961,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > if (unlikely(!s))
> > return;
> >
> > - flush_memcg_workqueue(s);
> > -
> > get_online_cpus();
> > get_online_mems();
> >
> > @@ -955,6 +970,30 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > if (s->refcount)
> > goto out_unlock;
> >
> > +#ifdef CONFIG_MEMCG_KMEM
> > + mutex_unlock(&slab_mutex);
> > +
> > + put_online_mems();
> > + put_online_cpus();
> > +
> > + flush_memcg_workqueue(s);
> > +
> > + get_online_cpus();
> > + get_online_mems();
> > +
> > + mutex_lock(&slab_mutex);
> > +
> > + if (WARN(s->refcount,
> > + "kmem_cache_destroy %s: Slab cache is still referenced\n",
> > + s->name)) {
> > + /*
> > + * Reset the dying flag setted by flush_memcg_workqueue().
> > + */
> > + s->memcg_params.dying = false;
> > + goto out_unlock;
> > + }
> > +#endif
> > +
> > err = shutdown_memcg_caches(s);
> > if (!err)
> > err = shutdown_cache(s);
> >
>
