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

[Andrew Morton]

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