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repart: make COW behavior configurable

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systemd-repart currently forces newly created image files into NOCOW mode.
That prevents files from being reflinked into the image, making image builds
slower and increasing their disk usage on filesystems that support cloning.

Add a tristate --cow= option. By default, leave the filesystem or parent
directory COW policy unchanged. With --cow=yes, explicitly enable COW; with
--cow=no, retain the previous behavior of forcing NOCOW. Add XO_COW as the
counterpart to XO_NOCOW so xopenat_full() applies either policy while retaining
its normal creation-error cleanup.

Document the new option and extend TEST-58-REPART to verify inherited COW and
NOCOW policies as well as explicit COW and NOCOW overrides. Compare the unset
behavior with the filesystem default so the test also works on nodatacow
mounts, and skip it when the inode attribute is unsupported.

Signed-off-by: Daan De Meyer <daan@amutable.com>
This commit is contained in:
Daan De Meyer
2026-07-15 09:22:58 +02:00
committed by Lennart Poettering
parent d153b49473
commit 28b1e4ea16
5 changed files with 140 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/basic/fs-util.c
+7 -5
View File
@@ -1183,12 +1183,13 @@ int xopenat_full(int dir_fd, const char *path, int open_flags, XOpenFlags xopen_
/* Sockets cannot be open()ed, only pinned via O_PATH. */
assert(!FLAGS_SET(xopen_flags, XO_SOCKET) || FLAGS_SET(open_flags, O_PATH));
/* XO_TRIGGER_AUTOMOUNT requires O_PATH and does not support creating inodes. XO_SUBVOLUME
* requires O_CREAT, and XO_NOCOW needs a writable fd for its chattr ioctl, so neither is
* requires O_CREAT, and XO_COW/XO_NOCOW need a writable fd for their chattr ioctl, so none are
* compatible with XO_TRIGGER_AUTOMOUNT. */
assert(!FLAGS_SET(xopen_flags, XO_TRIGGER_AUTOMOUNT) ||
(FLAGS_SET(open_flags, O_PATH) && !FLAGS_SET(open_flags, O_CREAT)));
assert(!(FLAGS_SET(xopen_flags, XO_TRIGGER_AUTOMOUNT) && FLAGS_SET(xopen_flags, XO_SUBVOLUME)));
assert(!(FLAGS_SET(xopen_flags, XO_TRIGGER_AUTOMOUNT) && FLAGS_SET(xopen_flags, XO_NOCOW)));
assert(!(FLAGS_SET(xopen_flags, XO_TRIGGER_AUTOMOUNT) && (xopen_flags & (XO_COW|XO_NOCOW))));
assert((xopen_flags & (XO_COW|XO_NOCOW)) != (XO_COW|XO_NOCOW));
/* Don't specify an access mode if you want auto mode. */
assert(!FLAGS_SET(xopen_flags, XO_AUTO_RW_RO) || (open_flags & O_ACCMODE_STRICT) == 0);
@@ -1202,7 +1203,8 @@ int xopenat_full(int dir_fd, const char *path, int open_flags, XOpenFlags xopen_
*
* • If the path is specified NULL or empty, behaves like fd_reopen().
*
* • If XO_NOCOW is specified will turn on the NOCOW btrfs flag on the file, if available.
* • If XO_COW or XO_NOCOW is specified will turn off or on the NOCOW btrfs flag on the file, if
* available.
*
* • if XO_REGULAR is specified will return an error if inode is not a regular file.
*
@@ -1436,8 +1438,8 @@ int xopenat_full(int dir_fd, const char *path, int open_flags, XOpenFlags xopen_
goto error;
}
if (FLAGS_SET(xopen_flags, XO_NOCOW)) {
r = chattr_fd(fd, FS_NOCOW_FL, FS_NOCOW_FL);
if (xopen_flags & (XO_COW|XO_NOCOW)) {
r = chattr_fd(fd, FLAGS_SET(xopen_flags, XO_NOCOW) ? FS_NOCOW_FL : 0, FS_NOCOW_FL);
if (r < 0 && !ERRNO_IS_IOCTL_NOT_SUPPORTED(r))
goto error;
}
src/basic/fs-util.h
+5 -4
View File
@@ -112,10 +112,11 @@ typedef enum XOpenFlags {
XO_LABEL = 1 << 0, /* When creating: relabel */
XO_SUBVOLUME = 1 << 1, /* When creating as directory: make it a subvolume */
XO_NOCOW = 1 << 2, /* Enable NOCOW mode after opening */
XO_REGULAR = 1 << 3, /* Fail if the inode is not a regular file */
XO_SOCKET = 1 << 4, /* Fail if the inode is not a socket */
XO_TRIGGER_AUTOMOUNT = 1 << 5, /* Trigger automounts via open_tree(). Requires O_PATH. */
XO_AUTO_RW_RO = 1 << 6, /* Open in O_RDWR mode if possible, O_RDONLY if not */
XO_COW = 1 << 3, /* Enable COW mode after opening */
XO_REGULAR = 1 << 4, /* Fail if the inode is not a regular file */
XO_SOCKET = 1 << 5, /* Fail if the inode is not a socket */
XO_TRIGGER_AUTOMOUNT = 1 << 6, /* Trigger automounts via open_tree(). Requires O_PATH. */
XO_AUTO_RW_RO = 1 << 7, /* Open in O_RDWR mode if possible, O_RDONLY if not */
} XOpenFlags;
int open_mkdir_at_full(int dirfd, const char *path, int flags, XOpenFlags xopen_flags, mode_t mode);