linux-kernel - Re: Why PAGEOUT_IO_SYNC stalls for a long time

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Message-Id: <20100728103056.c5511c78.akpm@linux-foundation.org>
Date:	Wed, 28 Jul 2010 10:30:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>, stable@...nel.org,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andreas Mohr <andi@...as.de>, Bill Davidsen <davidsen@....com>,
	Ben Gamari <bgamari.foss@...il.com>
Subject: Re: Why PAGEOUT_IO_SYNC stalls for a long time

On Wed, 28 Jul 2010 20:40:21 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> 3. pageout() is intended anynchronous api. but doesn't works so.
> 
> pageout() call ->writepage with wbc->nonblocking=1. because if the system have
> default vm.dirty_ratio (i.e. 20), we have 80% clean memory. so, getting stuck
> on one page is stupid, we should scan much pages as soon as possible.
> 
> HOWEVER, block layer ignore this argument. if slow usb memory device connect
> to the system, ->writepage() will sleep long time. because submit_bio() call
> get_request_wait() unconditionally and it doesn't have any PF_MEMALLOC task
> bonus.

The idea is that vmscan doesn't call ->writepage if the underlying
queue is congested.  may_write_to_queue()->bdi_queue_congested() should
return false and we skip the write.

If that logic is broken then that would explain a few things...
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