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

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Date:	Thu, 8 Mar 2012 18:33:14 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] memcg: avoid THP split in task migration

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.

And it's not how it has been and continues to behave with file pages.

Isn't getting that behaviour in fork-move-exec just a good reason not
to set move_charge_at_immigrate?

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().

I'd rather delete than add code here!

Hugh
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