smb3: fix performance regression with setting mtime

There are cases when we don't want to send the SMB2 flush operation
(e.g. when user specifies mount parm "nostrictsync") and it can be
a very expensive operation on the server.  In most cases in order
to set mtime, we simply need to flush (write) the dirtry pages from
the client and send the writes to the server not also send a flush
protocol operation to the server.

Fixes: aa081859 ("cifs: flush before set-info if we have writeable handles")
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/inode.c

@@ -2528,25 +2528,26 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs)
 
 	/*
 	 * Attempt to flush data before changing attributes. We need to do
-	 * this for ATTR_SIZE and ATTR_MTIME for sure, and if we change the
-	 * ownership or mode then we may also need to do this. Here, we take
-	 * the safe way out and just do the flush on all setattr requests. If
-	 * the flush returns error, store it to report later and continue.
+	 * this for ATTR_SIZE and ATTR_MTIME.  If the flush of the data
+	 * returns error, store it to report later and continue.
 	 *
 	 * BB: This should be smarter. Why bother flushing pages that
 	 * will be truncated anyway? Also, should we error out here if
-	 * the flush returns error?
+	 * the flush returns error? Do we need to check for ATTR_MTIME_SET flag?
 	 */
-	rc = filemap_write_and_wait(inode->i_mapping);
-	if (is_interrupt_error(rc)) {
-		rc = -ERESTARTSYS;
-		goto cifs_setattr_exit;
+	if (attrs->ia_valid & (ATTR_MTIME | ATTR_SIZE | ATTR_CTIME)) {
+		rc = filemap_write_and_wait(inode->i_mapping);
+		if (is_interrupt_error(rc)) {
+			rc = -ERESTARTSYS;
+			goto cifs_setattr_exit;
+		}
+		mapping_set_error(inode->i_mapping, rc);
 	}
 
-	mapping_set_error(inode->i_mapping, rc);
 	rc = 0;
 
-	if (attrs->ia_valid & ATTR_MTIME) {
+	if ((attrs->ia_valid & ATTR_MTIME) &&
+	    !(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_NOSSYNC)) {
 		rc = cifs_get_writable_file(cifsInode, FIND_WR_ANY, &wfile);
 		if (!rc) {
 			tcon = tlink_tcon(wfile->tlink);