[RFC 9/9] e2fsprogs/chattr: Supports atomic writes attribute

[Ritesh Harjani (IBM)]

This adds 'W' which is atomic write attribute to chattr.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
---
 lib/e2p/pf.c         |  1 +
 lib/ext2fs/ext2_fs.h |  2 +-
 misc/chattr.1.in     | 18 ++++++++++++++----
 misc/chattr.c        |  3 ++-
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/lib/e2p/pf.c b/lib/e2p/pf.c
index 81e3bb26..9b311477 100644
--- a/lib/e2p/pf.c
+++ b/lib/e2p/pf.c
@@ -45,6 +45,7 @@ static struct flags_name flags_array[] = {
 	{ EXT4_EXTENTS_FL, "e", "Extents" },
 	{ FS_NOCOW_FL, "C", "No_COW" },
 	{ FS_DAX_FL, "x", "DAX" },
+	{ FS_ATOMICWRITES_FL, "W", "ATOMIC_WRITES" },
 	{ EXT4_CASEFOLD_FL, "F", "Casefold" },
 	{ EXT4_INLINE_DATA_FL, "N", "Inline_Data" },
 	{ EXT4_PROJINHERIT_FL, "P", "Project_Hierarchy" },
diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
index 0fc9c09a..f9dcf71f 100644
--- a/lib/ext2fs/ext2_fs.h
+++ b/lib/ext2fs/ext2_fs.h
@@ -346,7 +346,7 @@ struct ext2_dx_tail {
 #define EXT4_EA_INODE_FL	        0x00200000 /* Inode used for large EA */
 /* EXT4_EOFBLOCKS_FL 0x00400000 was here */
 #define FS_NOCOW_FL			0x00800000 /* Do not cow file */
-#define EXT4_SNAPFILE_FL		0x01000000  /* Inode is a snapshot */
+#define FS_ATOMICWRITES_FL		0x01000000  /* Inode can do atomic writes */
 #define FS_DAX_FL			0x02000000 /* Inode is DAX */
 #define EXT4_SNAPFILE_DELETED_FL	0x04000000  /* Snapshot is being deleted */
 #define EXT4_SNAPFILE_SHRUNK_FL		0x08000000  /* Snapshot shrink has completed */
diff --git a/misc/chattr.1.in b/misc/chattr.1.in
index 50c54e7d..22757123 100644
--- a/misc/chattr.1.in
+++ b/misc/chattr.1.in
@@ -26,7 +26,7 @@ changes the file attributes on a Linux file system.
 The format of a symbolic
 .I mode
 is
-.BR +-= [ aAcCdDeFijmPsStTux ].
+.BR +-= [ aAcCdDeFijmPsStTuxW ].
 .PP
 The operator
 .RB ' + '
@@ -38,7 +38,7 @@ causes them to be removed; and
 causes them to be the only attributes that the files have.
 .PP
 The letters
-.RB ' aAcCdDeFijmPsStTux '
+.RB ' aAcCdDeFijmPsStTuxW '
 select the new attributes for the files:
 append only
 .RB ( a ),
@@ -74,8 +74,10 @@ top of directory hierarchy
 .RB ( T ),
 undeletable
 .RB ( u ),
-and direct access for files
-.RB ( x ).
+direct access for files
+.RB ( x ),
+and atomic writes for files.
+.RB ( W ).
 .PP
 The following attributes are read-only, and may be listed by
 .BR lsattr (1)
@@ -263,6 +265,14 @@ directory.  If an existing directory has contained some files and
 subdirectories, modifying the attribute on the parent directory doesn't
 change the attributes on these files and subdirectories.
 .TP
+.B W
+The 'W' attribute can only be set on a regular file. A file which has this
+attribute set can do untorn writes i.e. if an atomic write is requested by
+user with proper alignment and atomic flags set (such as RWF_ATOMIC), then
+a subsequent read to that block(s) will either read entire new data or entire
+old data (in case of a power failure). The block(s) written can never contain
+mix of both.
+.TP
 .B V
 A file with the 'V' attribute set has fs-verity enabled.  It cannot be
 written to, and the file system will automatically verify all data read
diff --git a/misc/chattr.c b/misc/chattr.c
index c7382a37..24db790e 100644
--- a/misc/chattr.c
+++ b/misc/chattr.c
@@ -86,7 +86,7 @@ static unsigned long sf;
 static void usage(void)
 {
 	fprintf(stderr,
-		_("Usage: %s [-RVf] [-+=aAcCdDeijPsStTuFx] [-p project] [-v version] files...\n"),
+		_("Usage: %s [-RVf] [-+=aAcCdDeijPsStTuFxW] [-p project] [-v version] files...\n"),
 		program_name);
 	exit(1);
 }
@@ -114,6 +114,7 @@ static const struct flags_char flags_array[] = {
 	{ EXT2_TOPDIR_FL, 'T' },
 	{ FS_NOCOW_FL, 'C' },
 	{ FS_DAX_FL, 'x' },
+	{ FS_ATOMICWRITES_FL, 'W' },
 	{ EXT4_CASEFOLD_FL, 'F' },
 	{ 0, 0 }
 };
--
2.39.2