On Wed, Jan 19, 2022 at 5:33 PM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > On Tue, Jan 18, 2022 at 08:05:44PM +0800, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote: > > I have thought about this. It's a little different to rely on objcg > > reparenting since the user can get memcg from objcg and > > then does not realize the memcg has reparented. > > When you pointed that out, I'm now also wondering how > memcg_list_lru_alloc() would be synchronized against > reparenting/renumbering of kmemcg_ids. What I suspect is that newly > allocated mlru may be stored into the xarray with a stale kmemcg_id. The synchronization is based on the lock of list_lru->lock. memcg_list_lru_free() will help us do housekeeping. > > > Maybe holding css_set_lock can do that. I do not think this > > is a good choice. > > I agree, it doesn't sound well. > > > Do you have any thoughts about this? > > Thoughts / questions of what I don't undestand well: > - Why do you allocate mlrus for all ancestors in memcg_list_lru_alloc()? It's because we need to be reparenting. > - It'd be sufficient to allocate just for the current memcg. > - Possibly allocate ancestors upon reparenting (to simplify the > allocation from slab_pre_alloc_hook itself). I agree it is nice to only allocate for current memcg, but reparenting cannot handle failure of memory allocation. > > - What is the per-kmemcg_id lookup good for? > - I observe most calls of list_lru_from_memcg_idx() come from callers > that know memcg (or even objcg). > - The non-specific use case seems list_lru_walk_node() working with > per-node and not per-memcg projection. > - Consequently that is only used over all nodes anyway > (list_lru_walk(). > - The idea behind this question is -- attach the list_lrus to > obj_cgroup (and decommission the kmemcg_id completely). > (Not necessarily part of this series but independent approach.) > I have some questions about this thought. We would attach more than one list_lrus to obj_cgroup, right? How to arrange those list_lrus, array or linked-list? Thanks.
