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Date:   Thu, 2 Jul 2020 09:37:38 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Michal Hocko <mhocko@kernel.org>
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Subject: Re: BUG: Bad page state in process - page dumped because: page still
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On Thu, Jul 02, 2020 at 06:22:02PM +0200, Michal Hocko wrote:
> On Wed 01-07-20 11:45:52, Roman Gushchin wrote:
> [...]
> > >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001
> > From: Roman Gushchin <guro@fb.com>
> > Date: Wed, 1 Jul 2020 11:05:32 -0700
> > Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible
> > 
> > Historically the kernel memory accounting was an opt-in feature, which
> > could be enabled for individual cgroups. But now it's not true, and
> > it's on by default both on cgroup v1 and cgroup v2.  And as long as a
> > user has at least one non-root memory cgroup, the kernel memory
> > accounting is on. So in most setups it's either always on (if memory
> > cgroups are in use and kmem accounting is not disabled), either always
> > off (otherwise).
> > 
> > memcg_kmem_enabled() is used in many places to guard the kernel memory
> > accounting code. If memcg_kmem_enabled() can reverse from returning
> > true to returning false (as now), we can't rely on it on release paths
> > and have to check if it was on before.
> > 
> > If we'll make memcg_kmem_enabled() irreversible (always returning true
> > after returning it for the first time), it'll make the general logic
> > more simple and robust. It also will allow to guard some checks which
> > otherwise would stay unguarded.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > ---
> >  mm/memcontrol.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 50ae77f3985e..2d018a51c941 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3582,7 +3582,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg)
> >  	objcg->memcg = memcg;
> >  	rcu_assign_pointer(memcg->objcg, objcg);
> >  
> > -	static_branch_inc(&memcg_kmem_enabled_key);
> > +	if (!memcg_kmem_enabled())
> > +		static_branch_inc(&memcg_kmem_enabled_key);
> 
> Wouldn't be static_branch_enable() more readable?

Agree, will change, add reported-by and tested-by tags and resend.
Thanks!

Btw, don't we wanna to change memcg_kmem_enabled() definition
from static_branch_unlikely() to static_branch_likely()?