Re: [PATCH 12/26] xfs: reject all unaligned direct writes to reflinked files

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On Sat, Apr 01, 2017 at 07:21:08PM +0200, Greg KH wrote:
> On Sat, Apr 01, 2017 at 08:34:58AM +0200, Christoph Hellwig wrote:
> > commit 54a4ef8af4e0dc5c983d17fcb9cf5fd25666d94e upstream.
> > 
> > We currently fall back from direct to buffered writes if we detect a
> > remaining shared extent in the iomap_begin callback.  But by the time
> > iomap_begin is called for the potentially unaligned end block we might
> > have already written most of the data to disk, which we'd now write
> > again using buffered I/O.  To avoid this reject all writes to reflinked
> > files before starting I/O so that we are guaranteed to only write the
> > data once.
> > 
> > The alternative would be to unshare the unaligned start and/or end block
> > before doing the I/O. I think that's doable, and will actually be
> > required to support reflinks on DAX file system.  But it will take a
> > little more time and I'd rather get rid of the double write ASAP.
> > 
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> > Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> 
> Any specific reason you don't want this one in 4.9 as well?

Nevermind, it's there, missed it, my fault...

greg k-h
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