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Date: Wed, 3 Aug 2016 13:09:42 +0200 From: Michal Hocko <mhocko@...nel.org> To: Vladimir Davydov <vdavydov@...tuozzo.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, stable@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup On Wed 03-08-16 12:50:49, Vladimir Davydov wrote: > On Tue, Aug 02, 2016 at 06:00:26PM +0200, Michal Hocko wrote: > > On Tue 02-08-16 18:00:48, Vladimir Davydov wrote: > ... > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 3be791afd372..4ae12effe347 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -4036,6 +4036,24 @@ static void mem_cgroup_id_get(struct mem_cgroup *memcg) > > > atomic_inc(&memcg->id.ref); > > > } > > > > > > +static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg) > > > +{ > > > + while (!atomic_inc_not_zero(&memcg->id.ref)) { > > > + /* > > > + * The root cgroup cannot be destroyed, so it's refcount must > > > + * always be >= 1. > > > + */ > > > + if (memcg == root_mem_cgroup) { > > > + VM_BUG_ON(1); > > > + break; > > > + } > > > > why not simply VM_BUG_ON(memcg == root_mem_cgroup)? > > Because with DEBUG_VM disabled we could wind up looping forever here if > the refcount of the root_mem_cgroup got screwed up. On production > kernels, it's better to break the loop and carry on closing eyes on > diverging counters rather than getting a lockup. Wouldn't this just paper over a real bug? Anyway I will not insist but making the code more complex just to pretend we can handle a situation gracefully doesn't sound right to me. -- Michal Hocko SUSE Labs
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