teach iomap_dio_rw() to suppress dsync

New flag, equivalent to removal of IOCB_DSYNC from iocb flags.
This mimics what btrfs is doing (and that's what btrfs will
switch to).  However, I'm not at all sure that we want to
suppress REQ_FUA for those - all btrfs hack really cares about
is suppression of generic_write_sync().  For now let's keep
the existing behaviour, but I really want to hear more detailed
arguments pro or contra.

[folded brain fix from willy]
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/iomap/direct-io.c

@@ -548,17 +548,19 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		}
 
 		/* for data sync or sync, we need sync completion processing */
-		if (iocb->ki_flags & IOCB_DSYNC)
+		if (iocb->ki_flags & IOCB_DSYNC &&
+		    !(dio_flags & IOMAP_DIO_NOSYNC)) {
 			dio->flags |= IOMAP_DIO_NEED_SYNC;
 
-		/*
-		 * For datasync only writes, we optimistically try using FUA for
-		 * this IO.  Any non-FUA write that occurs will clear this flag,
-		 * hence we know before completion whether a cache flush is
-		 * necessary.
-		 */
-		if ((iocb->ki_flags & (IOCB_DSYNC | IOCB_SYNC)) == IOCB_DSYNC)
-			dio->flags |= IOMAP_DIO_WRITE_FUA;
+		       /*
+			* For datasync only writes, we optimistically try
+			* using FUA for this IO.  Any non-FUA write that
+			* occurs will clear this flag, hence we know before
+			* completion whether a cache flush is necessary.
+			*/
+			if (!(iocb->ki_flags & IOCB_SYNC))
+				dio->flags |= IOMAP_DIO_WRITE_FUA;
+		}
 	}
 
 	if (dio_flags & IOMAP_DIO_OVERWRITE_ONLY) {

include/linux/iomap.h

@@ -353,6 +353,12 @@ struct iomap_dio_ops {
  */
 #define IOMAP_DIO_PARTIAL		(1 << 2)
 
+/*
+ * The caller will sync the write if needed; do not sync it within
+ * iomap_dio_rw.  Overrides IOMAP_DIO_FORCE_WAIT.
+ */
+#define IOMAP_DIO_NOSYNC		(1 << 3)
+
 ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
 		unsigned int dio_flags, void *private, size_t done_before);