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Date:   Wed, 22 Jan 2020 13:31:13 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Alex Shi <alex.shi@linux.alibaba.com>
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Subject: Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock
Message-ID: <20200122183113.GA98452@cmpxchg.org>
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 <9ee80b68-a78f-714a-c727-1f6d2b4f87ea@linux.alibaba.com>
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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)