smb3: add mount option to allow forced caching of read only share

If a share is immutable (at least for the period that it will
be mounted) it would be helpful to not have to revalidate
dentries repeatedly that we know can not be changed remotely.

Add "cache=" option (cache=ro) for mounting read only shares
in order to improve performance in cases in which we know that
the share will not be changing while it is in use.
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifs_fs_sb.h

@@ -53,6 +53,7 @@
 #define CIFS_MOUNT_NO_HANDLE_CACHE 0x4000000 /* disable caching dir handles */
 #define CIFS_MOUNT_NO_DFS 0x8000000 /* disable DFS resolving */
 #define CIFS_MOUNT_MODE_FROM_SID 0x10000000 /* retrieve mode from special ACE */
+#define CIFS_MOUNT_RO_CACHE	0x20000000  /* assumes share will not change */
 
 struct cifs_sb_info {
 	struct rb_root tlink_tree;

fs/cifs/cifsfs.c

@@ -400,6 +400,8 @@ cifs_show_cache_flavor(struct seq_file *s, struct cifs_sb_info *cifs_sb)
 		seq_puts(s, "strict");
 	else if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_DIRECT_IO)
 		seq_puts(s, "none");
+	else if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_RO_CACHE)
+		seq_puts(s, "ro"); /* read only caching assumed */
 	else
 		seq_puts(s, "loose");
 }

fs/cifs/cifsglob.h

@@ -559,6 +559,7 @@ struct smb_vol {
 	bool server_ino:1; /* use inode numbers from server ie UniqueId */
 	bool direct_io:1;
 	bool strict_io:1; /* strict cache behavior */
+	bool cache_ro:1;
 	bool remap:1;      /* set to remap seven reserved chars in filenames */
 	bool sfu_remap:1;  /* remap seven reserved chars ala SFU */
 	bool posix_paths:1; /* unset to not ask for posix pathnames. */
@@ -620,7 +621,8 @@ struct smb_vol {
 			 CIFS_MOUNT_MULTIUSER | CIFS_MOUNT_STRICT_IO | \
 			 CIFS_MOUNT_CIFS_BACKUPUID | CIFS_MOUNT_CIFS_BACKUPGID | \
 			 CIFS_MOUNT_UID_FROM_ACL | CIFS_MOUNT_NO_HANDLE_CACHE | \
-			 CIFS_MOUNT_NO_DFS | CIFS_MOUNT_MODE_FROM_SID)
+			 CIFS_MOUNT_NO_DFS | CIFS_MOUNT_MODE_FROM_SID | \
+			 CIFS_MOUNT_RO_CACHE)
 
 /**
  * Generic VFS superblock mount flags (s_flags) to consider when
@@ -1366,7 +1368,7 @@ void cifsFileInfo_put(struct cifsFileInfo *cifs_file);
 #define CIFS_CACHE_RW_FLG	(CIFS_CACHE_READ_FLG | CIFS_CACHE_WRITE_FLG)
 #define CIFS_CACHE_RHW_FLG	(CIFS_CACHE_RW_FLG | CIFS_CACHE_HANDLE_FLG)
 
-#define CIFS_CACHE_READ(cinode) (cinode->oplock & CIFS_CACHE_READ_FLG)
+#define CIFS_CACHE_READ(cinode) ((cinode->oplock & CIFS_CACHE_READ_FLG) || (CIFS_SB(cinode->vfs_inode.i_sb)->mnt_cifs_flags & CIFS_MOUNT_RO_CACHE))
 #define CIFS_CACHE_HANDLE(cinode) (cinode->oplock & CIFS_CACHE_HANDLE_FLG)
 #define CIFS_CACHE_WRITE(cinode) (cinode->oplock & CIFS_CACHE_WRITE_FLG)
 

fs/cifs/connect.c

@@ -298,6 +298,7 @@ enum {
 	Opt_cache_loose,
 	Opt_cache_strict,
 	Opt_cache_none,
+	Opt_cache_ro,
 	Opt_cache_err
 };
 
@@ -305,6 +306,7 @@ static const match_table_t cifs_cacheflavor_tokens = {
 	{ Opt_cache_loose, "loose" },
 	{ Opt_cache_strict, "strict" },
 	{ Opt_cache_none, "none" },
+	{ Opt_cache_ro, "ro" },
 	{ Opt_cache_err, NULL }
 };
 
@@ -1418,14 +1420,22 @@ cifs_parse_cache_flavor(char *value, struct smb_vol *vol)
 	case Opt_cache_loose:
 		vol->direct_io = false;
 		vol->strict_io = false;
+		vol->cache_ro = false;
 		break;
 	case Opt_cache_strict:
 		vol->direct_io = false;
 		vol->strict_io = true;
+		vol->cache_ro = false;
 		break;
 	case Opt_cache_none:
 		vol->direct_io = true;
 		vol->strict_io = false;
+		vol->cache_ro = false;
+		break;
+	case Opt_cache_ro:
+		vol->direct_io = false;
+		vol->strict_io = false;
+		vol->cache_ro = true;
 		break;
 	default:
 		cifs_dbg(VFS, "bad cache= option: %s\n", value);
@@ -4041,6 +4051,10 @@ int cifs_setup_cifs_sb(struct smb_vol *pvolume_info,
 		cifs_dbg(FYI, "mounting share using direct i/o\n");
 		cifs_sb->mnt_cifs_flags |= CIFS_MOUNT_DIRECT_IO;
 	}
+	if (pvolume_info->cache_ro) {
+		cifs_dbg(VFS, "mounting share with read only caching. Ensure that the share will not be modified while in use.\n");
+		cifs_sb->mnt_cifs_flags |= CIFS_MOUNT_RO_CACHE;
+	}
 	if (pvolume_info->mfsymlinks) {
 		if (pvolume_info->sfu_emul) {
 			/*