linux-kernel - Re: [PATCH] mm: disallow direct reclaim page writeback

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Message-Id: <20100415130212.D16E.A69D9226@jp.fujitsu.com>
Date:	Thu, 15 Apr 2010 13:09:01 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: disallow direct reclaim page writeback

Hi

> How about this? For now, we stop direct reclaim from doing writeback
> only on order zero allocations, but allow it for higher order
> allocations. That will prevent the majority of situations where
> direct reclaim blows the stack and interferes with background
> writeout, but won't cause lumpy reclaim to change behaviour.
> This reduces the scope of impact and hence testing and validation
> the needs to be done.

Tend to agree. but I would proposed slightly different algorithm for
avoind incorrect oom.

for high order allocation
	allow to use lumpy reclaim and pageout() for both kswapd and direct reclaim

for low order allocation
	- kswapd:          always delegate io to flusher thread
	- direct reclaim:  delegate io to flusher thread only if vm pressure is low

This seems more safely. I mean Who want see incorrect oom regression?
I've made some pathes for this. I'll post it as another mail.

> Then we can work towards allowing lumpy reclaim to use background
> threads as Chris suggested for doing specific writeback operations
> to solve the remaining problems being seen. Does this seem like a
> reasonable compromise and approach to dealing with the problem?

Tend to agree. probably now we are discussing right approach. but
this is definitely needed deep thinking. then, I can't take exactly
answer yet.




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