linux-kernel - Re: [PATCH v8 03/10] mm/lru: replace pgdat lru

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Message-ID: <20200122183113.GA98452@cmpxchg.org>
Date:   Wed, 22 Jan 2020 13:31:13 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        yang.shi@...ux.alibaba.com, willy@...radead.org,
        shakeelb@...gle.com, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Chris Down <chris@...isdown.name>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, Qian Cai <cai@....pw>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        swkhack <swkhack@...il.com>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Colin Ian King <colin.king@...onical.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Peng Fan <peng.fan@....com>,
        Nikolay Borisov <nborisov@...e.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock

On Wed, Jan 22, 2020 at 08:01:29PM +0800, Alex Shi wrote:
> Yes I understand isolatation would exclusive by PageLRU, but forgive my
> stupid, I didn't figure out how a new page lruvec adding could be blocked.

I don't see why we would need this. Can you elaborate where you think
this is a problem?

If compaction races with charging for example, compaction doesn't need
to prevent a new page from being added to an lruvec. PageLRU is only
set after page->mem_cgroup is updated, so there are two race outcomes:

1) TestClearPageLRU() fails. That means the page isn't (fully) created
yet and cannot be migrated. We goto isolate_fail before even trying to
lock the lruvec.

2) TestClearPageLRU() succeeds. That means the page was fully created
and page->mem_cgroup has been set up. Anybody who now wants to change
page->mem_cgroup needs PageLRU, but we have it, so lruvec is stable.

I.e. cgroup charging does this:

	page->mem_cgroup = new_group

	lock(pgdat->lru_lock)
	SetPageLRU()
	add_page_to_lru_list()
	unlock(pgdat->lru_lock)

and compaction currently does this:

	lock(pgdat->lru_lock)
	if (!PageLRU())
		goto isolate_fail
	// __isolate_lru_page:
	if (!get_page_unless_zero())
		goto isolate_fail
	ClearPageLRU()
	del_page_from_lru_list()
	unlock(pgdat->lru_lock)

We can replace charging with this:

	page->mem_cgroup = new_group

	lock(lruvec->lru_lock)
	add_page_to_lru_list()
	unlock(lruvec->lru_lock)

	SetPageLRU()

and the compaction sequence with something like this:

	if (!get_page_unless_zero())
		goto isolate_fail

	if (!TestClearPageLRU())
		goto isolate_fail_put

	// We got PageLRU, so charging is complete and nobody
	// can modify page->mem_cgroup until we set it again.

	lruvec = mem_cgroup_page_lruvec(page, pgdat)
	lock(lruvec->lru_lock)
	del_page_from_lru_list()
	unlock(lruvec->lru_lock)