linux-kernel - Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

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Date:	Mon, 14 Jun 2010 18:45:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

On Mon, 14 Jun 2010 21:16:29 -0400 Rik van Riel <riel@...hat.com> wrote:

> Would it be hard to add a "please flush this file"
> way to call the filesystem flushing threads?

Passing the igrab()bed inode into the flusher threads would fix the
iput_final() problems, as long as the alloc_pages() caller never blocks
indefinitely waiting for the work which the flusher threads are doing. 

Otherwise we get (very hard-to-hit) deadlocks where the alloc_pages()
caller holds VFS locks and is waiting for the flusher threads while all
the flusher threads are stuck under iput_final() waiting for those VFS
locks.

That's fixable by not using igrab()/iput().  You can use lock_page() to
pin the address_space.  Pass the address of the locked page across to
the flusher threads so they don't try to lock it a second time, or just
use trylocking on that writeback path or whatever.

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