Re: [RFC][RFT] memcg fix cgroup_mutex deadlock when cpuset reclaims memory

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2008-12-10 16:41:26]:

> On Wed, 10 Dec 2008 15:19:48 +0900, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > On Wed, 10 Dec 2008 10:49:47 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > Hi,
> > > 
> > > Here is a proposed fix for the memory controller cgroup_mutex deadlock
> > > reported. It is lightly tested and reviewed. I need help with review
> > > and test. Is the reported deadlock reproducible after this patch? A
> > > careful review of the cpuset impact will also be highly appreciated.
> > > 
> > > From: Balbir Singh <balbir@linux.vnet.ibm.com>
> > > 
> > > cpuset_migrate_mm() holds cgroup_mutex throughout the duration of
> > > do_migrate_pages(). The issue with that is that
> > > 
> > > 1. It can lead to deadlock with memcg, as do_migrate_pages()
> > >    enters reclaim
> > > 2. It can lead to long latencies, preventing users from creating/
> > >    destroying other cgroups anywhere else
> > > 
> > > The patch holds callback_mutex through the duration of cpuset_migrate_mm() and
> > > gives up cgroup_mutex while doing so.
> > > 
> > I agree changing cpuset_migrate_mm not to hold cgroup_mutex to fix the dead lock
> > is one choice, and it looks good to me at the first impression.
> > 
> > But I'm not sure it's good to change cpuset(other subsystem) code because of memcg.
> > 
> > Anyway, I'll test this patch and report the result tomorrow.
> > (Sorry, I don't have enough time today.)
> > 
> Unfortunately, this patch doesn't seem enough.
> 
> This patch can fix dead lock caused by "circular lock of cgroup_mutex",
> but cannot that of caused by "race between page reclaim and cpuset_attach(mpol_rebind_mm)".
> 
> (The dead lock I fixed in memcg-avoid-dead-lock-caused-by-race-between-oom-and-cpuset_attach.patch
> was caused by "race between memcg's oom and mpol_rebind_mm, and was independent of hierarchy.)
> 

Yes, I agree, my point was to fix the deadlock caused in the hierarchy
due to cpuset_migrate_mm(). If I understand correctly

1. This patch introduces no new bug, but the old bug remains. The
deadlock is fixed
2. We need this patch + your fix to completely solve the problem?

Could you also share how to reproduce the issue, I'll test on my end
as well.

Thanks for your help!

-- 
	Balbir

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