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Date: Tue, 9 Apr 2013 14:04:32 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Jan Kara <jack@suse.cz>, Mel Gorman <mgorman@suse.de>,
        Shuge <shugelinux@gmail.com>, linux-kernel@vger.kernel.org,
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        Kevin <kevin@allwinnertech.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Jens Axboe <axboe@kernel.dk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3] [RESEND] mm: Make snapshotting pages for stable
 writes a per-bio operation
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On Tue, 9 Apr 2013 11:06:17 -0700 "Darrick J. Wong" <darrick.wong@oracle.com> wrote:

> +		 * Here we write back pagecache data that may be mmaped. Since
> +		 * we cannot afford to clean the page and set PageWriteback
> +		 * here due to lock ordering (page lock ranks above transaction
> +		 * start), the data can change while IO is in flight. Tell the
> +		 * block layer it should bounce the bio pages if stable data
> +		 * during write is required.

I think there are already ab/ba deadlocks between lock_page() and
journal_start().  iirc one path was write(), I forget which was the
other path.  This was 10+ years ago and nobody else noticed and I
didn't know how to fix it so I didn't tell anyone ;)

It would be neat to be able to hook things like journal_start() into
lockdep but I don't think that lockdep has easy provision for wiring
oddball things into its mechanisms.