xfs: reject all unaligned direct writes to reflinked files

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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@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

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Christoph Hellwig 2017-02-06 13:00:54 -08:00 • committed by Darrick J. Wong

parent 4dd2eb6335

commit 54a4ef8af4

3 changed files with 11 additions and 12 deletions

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9

fs/xfs/xfs_file.c

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12

fs/xfs/xfs_iomap.c

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2

fs/xfs/xfs_trace.h

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