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

From: Andrew Morton
Date: Tue Apr 09 2013 - 17:04:40 EST

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