Re: [PATCH v3 2/2] memcg: avoid THP split in task migration

[KAMEZAWA Hiroyuki]

On Fri, 9 Mar 2012 12:24:48 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Thu, 8 Mar 2012 18:33:14 -0800 (PST)
> Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> > On Fri, 9 Mar 2012, KAMEZAWA Hiroyuki wrote:
> > > > +
> > > > +	page = pmd_page(pmd);
> > > > +	VM_BUG_ON(!page || !PageHead(page));
> > > > +	if (!move_anon() || page_mapcount(page) != 1)
> > > > +		return 0;
> > > 
> > > Could you add this ?
> > > ==
> > > static bool move_check_shared_map(struct page *page)
> > > {
> > >   /*
> > >    * Handling of shared pages between processes is a big trouble in memcg.
> > >    * Now, we never move shared-mapped pages between memcg at 'task' moving because
> > >    * we have no hint which task the page is really belongs to. For example, 
> > >    * When a task does "fork()-> move to the child other group -> exec()", the charges
> > >    * should be stay in the original cgroup. 
> > >    * So, check mapcount to determine we can move or not.
> > >    */
> > >    return page_mapcount(page) != 1;
> > > }
> > 
> > That's a helpful elucidation, thank you.  However...
> > 
> > That is not how it has actually been behaving for the last 18 months
> > (because of the "> 2" bug), so in practice you are asking for a change
> > in behaviour there.
> > 
> Yes.
> 
> 
> > And it's not how it has been and continues to behave with file pages.
> > 
> It's ok to add somethink like..
> 
> 	if (PageAnon(page) && !move_anon())
> 		return false;
> 	...
> 
> > Isn't getting that behaviour in fork-move-exec just a good reason not
> > to set move_charge_at_immigrate?
> > 
> Hmm. Maybe.
> 
> > I think there are other scenarios where you do want all the pages to
> > move if move_charge_at_immigrate: and that's certainly easier to
> > describe and to understand and to code.
> > 
> > But if you do insist on not moving the shared, then it needs to involve
> > something like mem_cgroup_count_swap_user() on PageSwapCache pages,
> > rather than just the bare page_mapcount().
> > 
> 
> This 'moving swap account' was a requirement from a user (NEC?).
> But no user doesn't say 'I want to move shared pages between cgroups at task
> move !' and I don't like to move shared objects.
> 
> > I'd rather delete than add code here!
> > 
> 
> As a user, for Fujitsu, I believe it's insane to move task between cgroups.
> So, I have no benefit from this code, at all.
> Ok, maybe I'm not a stakeholder,here.
> 

Considering again, in my personal opinion, 
at fork() -> move() -> exec(), parent's RSS charge should not be moved.

Thanks,
-Kame

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