Re: [BUG] fs/super: a possible sleep-in-atomic bug in put_super

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Sun, Oct 08, 2017 at 03:03:32AM +0100, Al Viro wrote:
> On Sun, Oct 08, 2017 at 01:56:08AM +0100, Al Viro wrote:
> 
> > What's more, we need to be careful about resize vs. drain.  Right now it's
> > on list_lrus_mutex, but if we drop that around actual resize of an individual
> > list_lru, we'll need something else.  Would there be any problem if we
> > took memcg_cache_ids_sem shared in memcg_offline_kmem()?
> > 
> > The first problem is not fatal - we can e.g. use the sign of the field used
> > to store the number of ->memcg_lrus elements (i.e. stashed value of
> > memcg_nr_cache_ids at allocation or last resize) to indicate that actual
> > freeing is left for resizer...
> 
> Ugh.  That spinlock would have to be held over too much work, or bounced back
> and forth a lot on memcg shutdowns ;-/  Gets especially nasty if we want
> list_lru_destroy() callable from rcu callbacks.  Oh, well...
> 
> I still suspect that locking there is too heavy, but it looks like I don't have
> a better replacement.
> 
> What are the realistic numbers of memcg on a big system?

Several thousand. I guess we could turn list_lrus_mutex into a spin lock
by making resize/drain procedures handle list_lru destruction as you
suggested above, but list_lru_destroy() would still have to iterate over
all elements of list_lru_node->memcg_lrus array to free per-memcg
objects, which is too heavy to be performed under sb_lock IMHO.

Thanks,
Vladimir