linux-kernel - Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting

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Date:   Fri, 24 Aug 2018 08:42:11 -0700
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        Andy Lutomirski <luto@...nel.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Tejun Heo <tj@...nel.org>, Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting

On Fri, Aug 24, 2018 at 08:50:52AM -0400, Johannes Weiner wrote:
> 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.

Fair enough. Will switch over hard-coded order 0 in v3.

Thanks!