fs: clean up the flags definition in uapi/linux/fs.h

Add an explanation for the flags used by FS_IOC_[GS]ETFLAGS and remind
people that changes should be revised by linux-fsdevel and linux-api.

Add flags that are used on-disk for ext4, and remove FS_DIRECTIO_FL
since it was used only by gfs2 and support was removed in 2008 in
commit c9f6a6bb ("The ability to mark files for direct i/o access
when opened normally is both unused and pointless, so this patch
removes support for that feature.")  Now we have _two_ remaining flags
left.  But since we want to discourage people from assigning new
flags, that's OK.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/uapi/linux/fs.h

@@ -2,8 +2,11 @@
 #define _UAPI_LINUX_FS_H
 
 /*
- * This file has definitions for some important file table
- * structures etc.
+ * This file has definitions for some important file table structures
+ * and constants and structures used by various generic file system
+ * ioctl's.  Please do not make any changes in this file before
+ * sending patches for review to linux-fsdevel@vger.kernel.org and
+ * linux-api@vger.kernel.org.
  */
 
 #include <linux/limits.h>
@@ -172,6 +175,23 @@ struct inodes_stat_t {
 
 /*
  * Inode flags (FS_IOC_GETFLAGS / FS_IOC_SETFLAGS)
+ *
+ * Note: for historical reasons, these flags were originally used and
+ * defined for use by ext2/ext3, and then other file systems started
+ * using these flags so they wouldn't need to write their own version
+ * of chattr/lsattr (which was shipped as part of e2fsprogs).  You
+ * should think twice before trying to use these flags in new
+ * contexts, or trying to assign these flags, since they are used both
+ * as the UAPI and the on-disk encoding for ext2/3/4.  Also, we are
+ * almost out of 32-bit flags.  :-)
+ *
+ * We have recently hoisted FS_IOC_FSGETXATTR / FS_IOC_FSSETXATTR from
+ * XFS to the generic FS level interface.  This uses a structure that
+ * has padding and hence has more room to grow, so it may be more
+ * appropriate for many new use cases.
+ *
+ * Please do not change these flags or interfaces before checking with
+ * linux-fsdevel@vger.kernel.org and linux-api@vger.kernel.org.
  */
 #define	FS_SECRM_FL			0x00000001 /* Secure deletion */
 #define	FS_UNRM_FL			0x00000002 /* Undelete */
@@ -185,8 +205,8 @@ struct inodes_stat_t {
 #define FS_DIRTY_FL			0x00000100
 #define FS_COMPRBLK_FL			0x00000200 /* One or more compressed clusters */
 #define FS_NOCOMP_FL			0x00000400 /* Don't compress */
-#define FS_ECOMPR_FL			0x00000800 /* Compression error */
 /* End compression flags --- maybe not all used */
+#define FS_ENCRYPT_FL			0x00000800 /* Encrypted file */
 #define FS_BTREE_FL			0x00001000 /* btree format dir */
 #define FS_INDEX_FL			0x00001000 /* hash-indexed directory */
 #define FS_IMAGIC_FL			0x00002000 /* AFS directory */
@@ -194,9 +214,12 @@ struct inodes_stat_t {
 #define FS_NOTAIL_FL			0x00008000 /* file tail should not be merged */
 #define FS_DIRSYNC_FL			0x00010000 /* dirsync behaviour (directories only) */
 #define FS_TOPDIR_FL			0x00020000 /* Top of directory hierarchies*/
+#define FS_HUGE_FILE_FL			0x00040000 /* Reserved for ext4 */
 #define FS_EXTENT_FL			0x00080000 /* Extents */
-#define FS_DIRECTIO_FL			0x00100000 /* Use direct i/o */
+#define FS_EA_INODE_FL			0x00200000 /* Inode used for large EA */
+#define FS_EOFBLOCKS_FL			0x00400000 /* Reserved for ext4 */
 #define FS_NOCOW_FL			0x00800000 /* Do not cow file */
+#define FS_INLINE_DATA_FL		0x10000000 /* Reserved for ext4 */
 #define FS_PROJINHERIT_FL		0x20000000 /* Create with parents projid */
 #define FS_RESERVED_FL			0x80000000 /* reserved for ext2 lib */