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

Ritesh Harjani (IBM)
2024-03-02T07:43:00Z

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

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
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