Re: BUG: Bad page state in process - page dumped because: page still charged to cgroup

[Roman Gushchin <guro@fb.com>]

On Thu, Jul 02, 2020 at 07:13:02PM +0200, Michal Hocko wrote:
> On Thu 02-07-20 09:37:38, Roman Gushchin wrote:
> > 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!
> 
> Feel free to add
> Acked-by: Michal Hocko <mhocko@suse.com>

Thank you!

> 
> > Btw, don't we wanna to change memcg_kmem_enabled() definition
> > from static_branch_unlikely() to static_branch_likely()?
> 
> Honestly, I do not know what would be the impact but considering kmem
> is enabled unless explicitly disabled these days then likely sounds more
> logical from reading POV. I do not think that early allocations until
> the first memcg is created is the case to optimize for.
> Worth a separate patch I guess.

Yeah, I doubt there will be any measurable difference, it just strained my eyes.
I prepare a small set of cleanups/cosmetic fixes, will add it to them.

Thanks!