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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 8 Mar 2012 18:33:14 -0800 (PST)
Hugh Dickins <hughd@google.com> 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.

If users say all shared pages should be moved, ok, let's move.
But change of behavior should be documented and implemented in an independet
patch. CC'ed Nishimura-san, he implemetned this, a real user.

Thanks,
-Kame



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