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

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Date:	Fri, 25 Jul 2014 13:34:06 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	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,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch 13/13] mm: memcontrol: rewrite uncharge API

On Fri, Jul 25, 2014 at 05:43:20PM +0200, Michal Hocko wrote:
> On Fri 25-07-14 11:26:54, Johannes Weiner wrote:
> > On Thu, Jul 24, 2014 at 11:02:57AM +0200, Michal Hocko wrote:
> > > On Thu 24-07-14 10:46:44, Michal Hocko wrote:
> > > > On Wed 23-07-14 17:02:41, Johannes Weiner wrote:
> > > [...]
> > > > We can reduce the lookup only to lruvec==true case, no?
> > > 
> > > Dohh
> > > s@can@...uld@
> > > 
> > > newpage shouldn't charged in all other cases and it would be bug.
> > > Or am I missing something?
> > 
> > Yeah, but I'd hate to put that assumption onto the @lrucare parameter,
> > it just coincides.
> 
> Yes, you are right. Maybe replace_page_cache_page should have it's own
> memcg variant which does all the trickery and then call
> mem_cgroup_migrate when necessary...

The code flow doesn't really lend itself to nesting.  It's basically
three steps: validate input, clear the old page, commit the new page.

void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
{
	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(PageAnon(oldpage) != PageAnon(newpage), newpage);
	VM_BUG_ON_PAGE(PageTransHuge(oldpage) != PageTransHuge(newpage),
		       newpage);

	if (mem_cgroup_disabled())
		return;

	/* Re-entrant migration: old page already uncharged? */
	pc = lookup_page_cgroup(oldpage);
	if (!PageCgroupUsed(pc))
		return;

	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);

	pc->flags = 0;
	commit_charge(newpage, pc->mem_cgroup, false);
}

void mem_cgroup_replace_page_cache(struct page *oldpage, struct page *newpage)
{
	struct page_cgroup *pc;
	int isolated;

	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);

	if (mem_cgroup_disabled())
		return;

	/* New page already charged? */
	pc = lookup_page_cgroup(newpage);
	if (PageCgroupUsed(pc))
		return;

	pc = lookup_page_cgroup(oldpage);

	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);

	lock_page_lru(oldpage, &isolated);
	pc->flags = 0;
	unlock_page_lru(oldpage, isolated);

	commit_charge(newpage, pc->mem_cgroup, true);
}

Only the call to commit_charge() is the same and there is a little bit
of overlap in the VM_BUG_ON_PAGEs...  I'd rather have a single migrate
function, because it's so small that the code is simpler than nesting
and/or duplicating multiple functions.
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