Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Jun 11, 2024 at 11:26:12AM GMT, Nhat Pham wrote:
[...]
> 
> Hmmm yeah in the past, I set it to NULL to make sure we're not
> replacing zswap_next_shrink with an offlined memcg, after that zswap
> offlining callback for that memcg has been completed..
> 
> I suppose we can just call mem_cgroup_iter(...) on that offlined
> cgroup, but I'm not 100% sure what happens when we call this function
> on a cgroup that is currently being offlined, and has gone past the
> zswap offline callback stage. So I was just playing it safe and
> restart from the top of the tree :)
> 
> I think this implementation has that behavior right? We see that the
> memcg is offlined, so we drop the lock and go to the beginning of the
> loop. We reacquire the lock, and might see that zswap_next_shrink ==
> memcg, so we call mem_cgroup_iter(...) on it. Is this safe?
> 
> Note that zswap_shrink_lock only orders serializes this memcg
> selection loop with memcg offlining after it - there's no guarantee
> what's the behavior is for memcg offlining before it (well other than
> one reference that we manage to acquire thanks to
> mem_cgroup_iter(...), so that memcg has not been freed, but not sure
> what we can guarantee regarding its place in the memcg hierarchy
> tree?).
> 
> Johannes, do you have any idea what we can expect here? Let me also cc Shakeel.
> 
> 
> 

mem_cgroup_iter() does a pre-order traversal, so you can see mixture of
online and offlined memcgs during a traversal.