linux-kernel - Re: [patch 3/3] mm: keep pages from unevictable mappings off the LRU lists

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Date:	Thu, 26 Mar 2009 01:48:38 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>
Subject: Re: [patch 3/3] mm: keep pages from unevictable mappings off the LRU  lists

On Mon, Mar 23, 2009 at 06:23:36PM +0900, KOSAKI Motohiro wrote:
> > > > this is the just reason why current code don't call add_page_to_unevictable_list().
> > > > add_page_to_unevictable_list() don't use pagevec. it is needed for avoiding race.
> > > > 
> > > > then, if readahead path (i.e. add_to_page_cache_lru()) use add_page_to_unevictable_list(),
> > > > it can cause zone->lru_lock contention storm.
> > > 
> > > How is it different then shrink_page_list()?  If readahead put a
> > > contiguous chunk of unevictable pages to the file lru, then
> > > shrink_page_list() will as well call add_page_to_unevictable_list() in
> > > a loop.
> > 
> > it's probability issue.
> > 
> > readahead: we need to concern
> > 	(1) readahead vs readahead
> > 	(2) readahead vs reclaim
> > 
> > vmscan: we need to concern
> > 	(3) background reclaim vs foreground reclaim
> > 
> > So, (3) is rarely event than (1) and (2).
> > Am I missing anything?
> 
> my last mail explanation is too poor. sorry.
> I don't dislike this patch concept. but it seems a bit naive against contention.
> if we can decrease contention risk, I can ack with presure.

My understanding is that when the mapping is truncated before the
pages are scanned for reclaim, then we have a net increase of risk for
the contention storm you describe.

Otherwise, we moved the contention from the reclaim path to the fault
path.

I don't know how likely readahead is.  It only happens when the
mapping was blown up with truncate, otherwise only writes add to the
cache in the ramfs case.

I will further look into this.

	Hannes
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