On Mon 16-12-13 23:09:23, Hugh Dickins wrote: > On Tue, 17 Dec 2013, Li Zefan wrote: > > On 2013/12/17 9:41, Hugh Dickins wrote: > > > On Mon, 16 Dec 2013, Tejun Heo wrote: > > >> On Mon, Dec 16, 2013 at 06:19:37PM +0100, Michal Hocko wrote: > > >>> I have to think about it some more (the brain is not working anymore > > >>> today). But what we really need is that nobody gets the same id while > > >>> the css is alive. > > > > That's what I meant to do in my last reply. > > > > But I'm confused by > > > > "How would this work? .. the swap will be there > > after the last reference to css as well." > > > > >>> So css_from_id returning NULL doesn't seem to be > > >>> enough. > > >> > > >> Oh, I meant whether it's necessary to keep css_from_id() working > > >> (ie. doing successful lookups) between offline and release, because > > >> that's where lifetimes are coupled. IOW, if it's enough for cgroup to > > >> not recycle the ID until all css's are released && fail css_from_id() > > >> lookup after the css is offlined, I can make a five liner quick fix. > > > > > > Don't take my word on it, I'm too fuzzy on this: but although it would > > > be good to refrain from recycling the ID until all css's are released, > > > I believe that it would not be good enough to fail css_from_id() once > > > the css is offlined - mem_cgroup_uncharge_swap() needs to uncharge the > > > hierarchy of the dead memcg (for example, when tmpfs file is removed). > > > > > > Uncharging the dead memcg itself is presumably irrelevant, but it does > > > need to locate the right parent to uncharge, and NULL css_from_id() > > > would make that impossible. It would be easy if we said those charges > > > migrate to root rather than to parent, but that's inconsistent with > > > what we have happily converged upon doing elsewhere (in the preferred > > > use_hierarchy case), and it would be a change in behaviour. > > > > > > I'm not nearly as enthusiastic for my patch as Michal is: I really > > > would prefer a five-liner from you or from Zefan. > > > > I've come up with a fix. Though it's more than five-line, it mostly moves > > a few lines from one place to another. I've tested it with your script. > > It seems to be working very well for me. I'm inclined to forgive you for > taking more than five lines, given that there are almost as many -s as +s ;) > > In my opinion, your patch is greatly preferable to mine - if there are > good things in mine, memcg can incorporate them at leisure later on, > but right now this seems a much better 3.13 solution. I'm guessing > Tejun and Hannes will feel the same way: how about you, Michal? OK, let's go with this for now but I would like swap accounting less tricky and confusing and explicit reparenting should help there I believe. And sorry for distracting you from the css based approach. I have totally misinterpreted the comment above idr_remove. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>