linux-kernel - Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

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Date:	Tue, 15 Jul 2014 14:19:35 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

[...]
> +/**
> + * mem_cgroup_migrate - migrate a charge to another page
> + * @oldpage: currently charged page
> + * @newpage: page to transfer the charge to
> + * @lrucare: page might be on LRU already

which one? I guess the newpage?

> + *
> + * Migrate the charge from @oldpage to @newpage.
> + *
> + * Both pages must be locked, @newpage->mapping must be set up.
> + */
> +void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
> +			bool lrucare)
> +{
> +	unsigned int nr_pages = 1;
> +	struct page_cgroup *pc;
> +
> +	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
> +	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
> +	VM_BUG_ON_PAGE(PageLRU(oldpage), oldpage);
> +	VM_BUG_ON_PAGE(PageLRU(newpage), newpage);

	VM_BUG_ON_PAGE(PageLRU(newpage) && !lruvec, newpage);

> +	VM_BUG_ON_PAGE(PageAnon(oldpage) != PageAnon(newpage), newpage);
> +
> +	if (mem_cgroup_disabled())
> +		return;
> +
> +	pc = lookup_page_cgroup(oldpage);
> +	if (!PageCgroupUsed(pc))
> +		return;
> +
> +	/* Already migrated */
> +	if (!(pc->flags & PCG_MEM))
> +		return;
> +
> +	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
> +	pc->flags &= ~(PCG_MEM | PCG_MEMSW);

What about PCG_USED?
Wouldn't we uncharge the currently transfered charge when oldpage does
its last put_page when the migration is done?

On a not directly related note. I was quite surprised to see that
__unmap_and_move calls putback_lru_page on oldpage even when migration
succeeded. So it goes through mem_cgroup_page_lruvec which checks
PCG_USED and resets pc->mem_cgroup to root for !PCG_USED.

> +
> +	if (PageTransHuge(oldpage)) {
> +		nr_pages <<= compound_order(oldpage);
> +		VM_BUG_ON_PAGE(!PageTransHuge(oldpage), oldpage);
> +		VM_BUG_ON_PAGE(!PageTransHuge(newpage), newpage);
> +	}
> +
> +	commit_charge(newpage, pc->mem_cgroup, nr_pages, lrucare);
> +}
-- 
Michal Hocko
SUSE Labs
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