Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg offline

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

On Fri, Sep 26, 2025 at 02:57:39PM +0800, Qi Zheng wrote:
> 
> 
> On 9/26/25 6:35 AM, Shakeel Butt wrote:
[...]
> > > 
> > > Yes, css_is_dying() can be used but please note that there is a rcu
> > > grace period between setting CSS_DYING and clearing CSS_ONLINE (i.e.
> > > reparenting deferred split queue) and during that period the deferred
> > > split THPs of the dying memcg will be hidden from shrinkers (which
> > > might be fine).
> 
> My mistake, now I think using css_is_dying() is safe.
> 
> > 
> > BTW if this period is not acceptable and we don't want to add is_dying
> > to struct deferred_split, we can use something similar to what list_lru
> > does in the similar situation i.e. set a special value (LONG_MIN) in its
> > nr_items variable. That is make split_queue_len a long and set it to
> > LONG_MIN during memcg offlining/reparenting.
> 
> I've considered this option, but I am concerned about the risk of
> overflow.

If you modify it only within a lock then why would it over or underflow?

> 
> So I will try to use css_is_dying() in the next version.

No objection from me but add a comment on the potential window where the
deferred thps will be hidden from the shrinkers.