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

[Hugh Dickins]

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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