iomap: use a srcmap for a read-modify-write I/O
The srcmap is used to identify where the read is to be performed from. It is passed to ->iomap_begin, which can fill it in if we need to read data for partially written blocks from a different location than the write target. The srcmap is only supported for buffered writes so far. Signed-off-by: NGoldwyn Rodrigues <rgoldwyn@suse.com> [hch: merged two patches, removed the IOMAP_F_COW flag, use iomap as srcmap if not set, adjust length down to srcmap end as well] Signed-off-by: NChristoph Hellwig <hch@lst.de> Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com> Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com> Acked-by: NGoldwyn Rodrigues <rgoldwyn@suse.com>
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