target/file: Re-enable optional fd_buffered_io=1 operation

This patch re-adds the ability to optionally run in buffered FILEIO mode
(eg: w/o O_DSYNC) for device backends in order to once again use the
Linux buffered cache as a write-back storage mechanism.

This logic was originally dropped with mainline v3.5-rc commit:

commit a4dff304
Author: Nicholas Bellinger <nab@linux-iscsi.org>
Date:   Wed May 30 16:25:41 2012 -0700

    target/file: Use O_DSYNC by default for FILEIO backends

This difference with this patch is that fd_create_virtdevice() now
forces the explicit setting of emulate_write_cache=1 when buffered FILEIO
operation has been enabled.

(v2: Switch to FDBD_HAS_BUFFERED_IO_WCE + add more detailed
     comment as requested by hch)
Reported-by: Ferry <iscsitmp@bananateam.nl>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

(cherry picked from commit b32f4c7e)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/target/target_core_file.c

@@ -117,6 +117,19 @@ static int fd_configure_device(struct se_device *dev)
 	 * of pure timestamp updates.
 	 */
 	flags = O_RDWR | O_CREAT | O_LARGEFILE | O_DSYNC;
+	/*
+	 * Optionally allow fd_buffered_io=1 to be enabled for people
+	 * who want use the fs buffer cache as an WriteCache mechanism.
+	 *
+	 * This means that in event of a hard failure, there is a risk
+	 * of silent data-loss if the SCSI client has *not* performed a
+	 * forced unit access (FUA) write, or issued SYNCHRONIZE_CACHE
+	 * to write-out the entire device cache.
+	 */
+	if (fd_dev->fbd_flags & FDBD_HAS_BUFFERED_IO_WCE) {
+		pr_debug("FILEIO: Disabling O_DSYNC, using buffered FILEIO\n");
+		flags &= ~O_DSYNC;
+	}
 
 	/*
 	 * Optionally allow fd_buffered_io=1 to be enabled for people
@@ -183,6 +196,12 @@ static int fd_configure_device(struct se_device *dev)
 		dev->dev_attrib.emulate_write_cache = 1;
 	}
 
+	if (fd_dev->fbd_flags & FDBD_HAS_BUFFERED_IO_WCE) {
+		pr_debug("FILEIO: Forcing setting of emulate_write_cache=1"
+			" with FDBD_HAS_BUFFERED_IO_WCE\n");
+		dev->se_sub_dev->se_dev_attrib.emulate_write_cache = 1;
+	}
+
 	fd_dev->fd_dev_id = fd_host->fd_host_dev_id_count++;
 	fd_dev->fd_queue_depth = dev->queue_depth;