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Date: Thu, 18 Jun 2015 13:12:27 +0200 From: Michal Hocko <mhocko@...e.cz> To: Tejun Heo <tj@...nel.org> Cc: axboe@...nel.dk, linux-kernel@...r.kernel.org, jack@...e.cz, hch@...radead.org, hannes@...xchg.org, linux-fsdevel@...r.kernel.org, vgoyal@...hat.com, lizefan@...wei.com, cgroups@...r.kernel.org, linux-mm@...ck.org, clm@...com, fengguang.wu@...el.com, david@...morbit.com, gthelen@...gle.com, khlebnikov@...dex-team.ru Subject: Re: [PATCH 06/51] memcg: add mem_cgroup_root_css On Wed 17-06-15 14:25:00, Tejun Heo wrote: > Hey, Michal. > > On Wed, Jun 17, 2015 at 04:56:42PM +0200, Michal Hocko wrote: > > On Fri 22-05-15 17:13:20, Tejun Heo wrote: > > > Add global mem_cgroup_root_css which points to the root memcg css. > > > > Is there any reason to using css rather than mem_cgroup other than the > > structure is not visible outside of memcontrol.c? Because I have a > > patchset which exports it. It is not merged yet so a move to mem_cgroup > > could be done later. I am just interested whether there is a stronger > > reason. > > It doesn't really matter either way but I think it makes a bit more > sense to use css as the common type when external code interacts with > cgroup controllers. e.g. cgroup writeback interacts with both memcg > and blkcg and in most cases it doesn't know or care about their > internal states. Most of what it wants is tracking them and doing > some common css operations (refcnting, printing and so on) on them. I see and yes, it makes some sense. I just think we can get rid of the accessor functions when the struct mem_cgroup is visible and the code can simply do &{page->}mem_cgroup->css. > > > This will be used by cgroup writeback support. If memcg is disabled, > > > it's defined as ERR_PTR(-EINVAL). > > > > Hmm. Why EINVAL? I can see only mm/backing-dev.c (in > > review-cgroup-writeback-switch-20150528 branch) which uses it and that > > shouldn't even try to compile if !CONFIG_MEMCG no? Otherwise we would > > simply blow up. > > Hmm... the code maybe has changed inbetween but there was something > which depended on the root css being defined when > !CONFIG_CGROUP_WRITEBACK or maybe it was on blkcg_root_css and memcg > side was added for consistency. I have tried to compile with !CONFIG_MEMCG and !CONFIG_CGROUP_WRITEBACK without mem_cgroup_root_css defined for this configuration and mm/backing-dev.c compiles just fine. So maybe we should get rid of it rather than have a potentially tricky code? > An ERR_PTR value is non-zero, which > is an invariant which is often depended upon, while guaranteeing oops > when deref'd. Yeah, but css_{get,put} and others consumers of the pointer are not checking for ERR_PTR. So I think this is really misleading. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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