linux-ext4 - Re: [PATCH v3] [RESEND] mm: Make snapshotting pages for stable writes a per-bio operation

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Date:	Tue, 9 Apr 2013 14:04:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Jan Kara <jack@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Shuge <shugelinux@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-ext4@...r.kernel.org,
	Kevin <kevin@...winnertech.com>, "Theodore Ts'o" <tytso@....edu>,
	Jens Axboe <axboe@...nel.dk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3] [RESEND] mm: Make snapshotting pages for stable
 writes a per-bio operation

On Tue, 9 Apr 2013 11:06:17 -0700 "Darrick J. Wong" <darrick.wong@...cle.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.

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