Re: [PATCH 1/5] iomap: FUA is wrong for DIO O_DSYNC writes into unwritten extents

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 20, 2018 at 08:17:38AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When we write into an unwritten extent via direct IO, we dirty
> metadata on IO completion to convert the unwritten extent to
> written. However, when we do the FUA optimisation checks, the inode
> may be clean and so we issue a FUA write into the unwritten extent.
> This means we then bypass the generic_write_sync() call after
> unwritten extent conversion has ben done and we don't force the
> modified metadata to stable storage.
> 
> This violates O_DSYNC semantics. The window of exposure is a single
> IO, as the next DIO write will see the inode has dirty metadata and
> hence will not use the FUA optimisation. Calling
> generic_write_sync() after completion of the second IO will also
> sync the first write and it's metadata.
> 
> Fix this by avoiding the FUA optimisation when writing to unwritten
> extents.

Ouch, yes.  We can't skip the log force when converting unwritten
extent.  If we really cared we could try to use FUA and only do
the log force vs needing a full device flush, but that would require
a fair amount of of work.

So this looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>