linux-kernel - Re: [PATCH 2/2] mlock use lru_add_drain_all

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Date:	Wed, 7 Oct 2009 12:48:31 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Ying Han <yinghan@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 2/2] mlock use lru_add_drain_all_async()

Hi

> Hello KOSAKI-san,
>
> Few questions on the lru_add_drain_all_async(). If i understand
> correctly, the reason that we have lru_add_drain_all() in the mlock()
> call is to isolate mlocked pages into the separate LRU in case they
> are sitting in pagevec.
>
> And I also understand the RT use cases you put in the patch
> description, now my questions is that do we have race after applying
> the patch? For example that if the RT task not giving up the cpu by
> the time mlock returns, you have pages left in the pagevec which not
> being drained back to the lru list. Do we have problem with that?

This patch don't introduce new race. current code has following race.

1. call mlock
2. lru_add_drain_all()
3. another cpu grab the page into its pagevec
4. actual PG_mlocked processing

I'd like to explain why this code works. linux has VM_LOCKED in vma
and PG_mlocked in page.  if we failed to turn on PG_mlocked, we can
recover it at vmscan phase by VM_LOCKED.

Then, this patch effect are
  - increase race possibility a bit
  - decrease RT-task problem risk

Thanks.
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