From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: Why PAGEOUT_IO_SYNC stalls for a long time
Date: Wed, 28 Jul 2010 10:30:56 -0700
Message-ID: <20100728103056.c5511c78.akpm@linux-foundation.org>
References: <20100728071705.GA22964@localhost>
	<20100728191322.4A85.A69D9226@jp.fujitsu.com>
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Cc: Wu Fengguang <fengguang.wu@intel.com>, stable@kernel.org,
        Rik van Riel
 <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>,
        Christoph Hellwig
 <hch@infradead.org>,
        "linux-kernel@vger.kernel.org"
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        "linux-fsdevel@vger.kernel.org"
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        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Dave Chinner <david@fromorbit.com>,
        Chris Mason <chris.mason@oracle.com>, Nick Piggin <npiggin@suse.de>,
        Johannes Weiner <hannes@cmpxchg.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Andrea Arcangeli
 <aarcange@redhat.com>,
        Minchan Kim <minchan.kim@gmail.com>, Andreas Mohr
 <andi@lisas.de>,
        Bill Davidsen <davidsen@tmr.com>, Ben Gamari
 <bgamari.foss@gmail.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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On Wed, 28 Jul 2010 20:40:21 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.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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