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Date:   Fri, 24 Aug 2018 08:50:52 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Andy Lutomirski <luto@kernel.org>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Tejun Heo <tj@kernel.org>, Shakeel Butt <shakeelb@google.com>
Subject: Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting
Message-ID: <20180824125052.GA13774@cmpxchg.org>
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On Thu, Aug 23, 2018 at 09:23:50AM -0700, Roman Gushchin wrote:
> On Wed, Aug 22, 2018 at 04:12:13PM +0200, Michal Hocko wrote:
> > On Tue 21-08-18 14:35:57, Roman Gushchin wrote:
> > > @@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> > >  static inline void free_thread_stack(struct task_struct *tsk)
> > >  {
> > >  #ifdef CONFIG_VMAP_STACK
> > > -	if (task_stack_vm_area(tsk)) {
> > > +	struct vm_struct *vm = task_stack_vm_area(tsk);
> > > +
> > > +	if (vm) {
> > >  		int i;
> > >  
> > > +		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
> > > +			mod_memcg_page_state(vm->pages[i],
> > > +					     MEMCG_KERNEL_STACK_KB,
> > > +					     -(int)(PAGE_SIZE / 1024));
> > > +
> > > +			memcg_kmem_uncharge(vm->pages[i],
> > > +					    compound_order(vm->pages[i]));
> > 
> > when do we have order > 0 here?
> 
> I guess, it's not possible, but hard-coded 1 looked a bit crappy.
> Do you think it's better?

Yes, specifying the known value (order 0) is much better. I asked
myself the same question as Michal: we're walking through THREAD_SIZE
in PAGE_SIZE steps, how could it possibly be a higher order page?

It adds an unnecessary branch to the code and the reader's brain.