[CIFS] Send SMB flush in cifs_fsync

In contrast to the now-obsolete smbfs, cifs does not send SMB_COM_FLUSH
in response to an explicit fsync(2) to guarantee that all volatile data
is written to stable storage on the server side, provided the server
honors the request (which, to my knowledge, is true for Windows and
Samba with 'strict sync' enabled).
This patch modifies the cifs_fsync implementation to restore the
fsync-behavior of smbfs by triggering SMB_COM_FLUSH after sending
outstanding data on the client side to the server.
Signed-off-by: Horst Reiterer <horst.reiterer@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/CHANGES

@@ -6,7 +6,9 @@ the server to treat subsequent connections, especially those that
 are authenticated as guest, as reconnections, invalidating the earlier
 user's smb session.  This fix allows cifs to mount multiple times to the
 same server with different userids without risking invalidating earlier
-established security contexts.
+established security contexts.  fsync now sends SMB Flush operation
+to better ensure that we wait for server to write all of the data to
+server disk (not just write it over the network).
 
 Version 1.56
 ------------

fs/cifs/cifs_debug.c

@@ -340,6 +340,8 @@ static int cifs_stats_proc_show(struct seq_file *m, void *v)
 				seq_printf(m, "\nWrites: %d Bytes: %lld",
 					atomic_read(&tcon->num_writes),
 					(long long)(tcon->bytes_written));
+				seq_printf(m, "\nFlushes: %d",
+					atomic_read(&tcon->num_flushes));
 				seq_printf(m, "\nLocks: %d HardLinks: %d "
 					      "Symlinks: %d",
 					atomic_read(&tcon->num_locks),

fs/cifs/cifsglob.h

@@ -254,6 +254,7 @@ struct cifsTconInfo {
 	atomic_t num_smbs_sent;
 	atomic_t num_writes;
 	atomic_t num_reads;
+	atomic_t num_flushes;
 	atomic_t num_oplock_brks;
 	atomic_t num_opens;
 	atomic_t num_closes;

fs/cifs/cifspdu.h

@@ -43,6 +43,7 @@
 #define SMB_COM_CREATE_DIRECTORY      0x00 /* trivial response */
 #define SMB_COM_DELETE_DIRECTORY      0x01 /* trivial response */
 #define SMB_COM_CLOSE                 0x04 /* triv req/rsp, timestamp ignored */
+#define SMB_COM_FLUSH                 0x05 /* triv req/rsp */
 #define SMB_COM_DELETE                0x06 /* trivial response */
 #define SMB_COM_RENAME                0x07 /* trivial response */
 #define SMB_COM_QUERY_INFORMATION     0x08 /* aka getattr */
@@ -790,6 +791,12 @@ typedef struct smb_com_close_rsp {
 	__u16 ByteCount;	/* bct = 0 */
 } __attribute__((packed)) CLOSE_RSP;
 
+typedef struct smb_com_flush_req {
+	struct smb_hdr hdr;	/* wct = 1 */
+	__u16 FileID;
+	__u16 ByteCount;	/* 0 */
+} __attribute__((packed)) FLUSH_REQ;
+
 typedef struct smb_com_findclose_req {
 	struct smb_hdr hdr; /* wct = 1 */
 	__u16 FileID;

fs/cifs/cifsproto.h

@@ -281,6 +281,9 @@ extern int CIFSPOSIXCreate(const int xid, struct cifsTconInfo *tcon,
 extern int CIFSSMBClose(const int xid, struct cifsTconInfo *tcon,
 			const int smb_file_id);
 
+extern int CIFSSMBFlush(const int xid, struct cifsTconInfo *tcon,
+			const int smb_file_id);
+
 extern int CIFSSMBRead(const int xid, struct cifsTconInfo *tcon,
 			const int netfid, unsigned int count,
 			const __u64 lseek, unsigned int *nbytes, char **buf,

fs/cifs/cifssmb.c

@@ -1933,6 +1933,27 @@ CIFSSMBClose(const int xid, struct cifsTconInfo *tcon, int smb_file_id)
 	return rc;
 }
 
+int
+CIFSSMBFlush(const int xid, struct cifsTconInfo *tcon, int smb_file_id)
+{
+	int rc = 0;
+	FLUSH_REQ *pSMB = NULL;
+	cFYI(1, ("In CIFSSMBFlush"));
+
+	rc = small_smb_init(SMB_COM_FLUSH, 1, tcon, (void **) &pSMB);
+	if (rc)
+		return rc;
+
+	pSMB->FileID = (__u16) smb_file_id;
+	pSMB->ByteCount = 0;
+	rc = SendReceiveNoRsp(xid, tcon->ses, (struct smb_hdr *) pSMB, 0);
+	cifs_stats_inc(&tcon->num_flushes);
+	if (rc)
+		cERROR(1, ("Send error in Flush = %d", rc));
+
+	return rc;
+}
+
 int
 CIFSSMBRename(const int xid, struct cifsTconInfo *tcon,
 	      const char *fromName, const char *toName,

fs/cifs/file.c

@@ -1523,6 +1523,9 @@ int cifs_fsync(struct file *file, struct dentry *dentry, int datasync)
 {
 	int xid;
 	int rc = 0;
+	struct cifsTconInfo *tcon;
+	struct cifsFileInfo *smbfile =
+		(struct cifsFileInfo *)file->private_data;
 	struct inode *inode = file->f_path.dentry->d_inode;
 
 	xid = GetXid();
@@ -1534,7 +1537,11 @@ int cifs_fsync(struct file *file, struct dentry *dentry, int datasync)
 	if (rc == 0) {
 		rc = CIFS_I(inode)->write_behind_rc;
 		CIFS_I(inode)->write_behind_rc = 0;
+		tcon = CIFS_SB(inode->i_sb)->tcon;
+		if (!rc && tcon && smbfile)
+			rc = CIFSSMBFlush(xid, tcon, smbfile->netfid);
 	}
+
 	FreeXid(xid);
 	return rc;
 }