cifs: only write 64kb at a time when fallocating a small region of a file

We only allow sending single credit writes through the SMB2_write() synchronous
api so split this into smaller chunks.

Fixes: 966a3cb ("cifs: improve fallocate emulation")

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reported-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Ronnie Sahlberg

fs/cifs/smb2ops.c

@@ -3617,21 +3617,33 @@ static int smb3_simple_fallocate_write_range(unsigned int xid,
 					     char *buf)
 {
 	struct cifs_io_parms io_parms = {0};
-	int nbytes;
+	int rc, nbytes;
 	struct kvec iov[2];
 
 	io_parms.netfid = cfile->fid.netfid;
 	io_parms.pid = current->tgid;
 	io_parms.tcon = tcon;
 	io_parms.persistent_fid = cfile->fid.persistent_fid;
 	io_parms.volatile_fid = cfile->fid.volatile_fid;
-	io_parms.offset = off;
-	io_parms.length = len;
 
-	/* iov[0] is reserved for smb header */
-	iov[1].iov_base = buf;
-	iov[1].iov_len = io_parms.length;
-	return SMB2_write(xid, &io_parms, &nbytes, iov, 1);
+	while (len) {
+		io_parms.offset = off;
+		io_parms.length = len;
+		if (io_parms.length > SMB2_MAX_BUFFER_SIZE)
+			io_parms.length = SMB2_MAX_BUFFER_SIZE;
+		/* iov[0] is reserved for smb header */
+		iov[1].iov_base = buf;
+		iov[1].iov_len = io_parms.length;
+		rc = SMB2_write(xid, &io_parms, &nbytes, iov, 1);
+		if (rc)
+			break;
+		if (nbytes > len)
+			return -EINVAL;
+		buf += nbytes;
+		off += nbytes;
+		len -= nbytes;
+	}
+	return rc;
 }
 
 static int smb3_simple_fallocate_range(unsigned int xid,