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

[Al Viro]

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?