Re: [RFC PATCH v3 1/5] vfs: add provenance time (ptime)

[Amir Goldstein]

On Tue, Nov 3, 2026 at 12:13 PM koraynilay <koray.fra@xxxxxxxxx> wrote:
>
> On Mon Nov 2, 2026 at 3:00 PM CET, Amir Goldstein wrote:
> > On Sun, Nov 1, 2026 at 2:57 AM koraynilay <koray.fra@xxxxxxxxx> wrote:
> >>
> >> Add provenance time (ptime), indicating the time when a file or
> >> directory was created. This is user-settable via file_setattr() and
> >> retrievable via statx(2) (using STATX_PTIME) or file_getattr().
> >>
> >> The semantics of when to preserve this new timestamp are the same as
> >> mtime, the idea being that there is a correspondence like this:
> >>
> >> ctime <-> btime
> >> mtime <-> ptime
> >>
> >> so every operation (copying, moving, ...) that currently preserves mtime
> >> should also preserve ptime.
> >>
> >> Furthermore, in order to avoid the legacy FS_IOC_FSSETXATTR accepting
> >> FS_XFLAG_PTIME, add an FS_FILE_ATTR_XFLAGS_MASK mask to support new
> >> xflags, and use it in fileattr_to_file_attr()/file_attr_to_fileattr() to
> >> allow ptime to be used with the file_setattr()/file_getattr() syscalls.
> >>
> >> [ This is a re-implementation of the initial patch[1] by Sean Smith
> >>   <defendthedisabled@xxxxxxxxx>, following the advice[2] from Darrick J.
> >>   Wong <djwong@xxxxxxxxxx> of using file_setattr() instead of updating
> >>   utimensat() ]
> >>
> >> [1]: https://lore.kernel.org/20260405195007.1306-2-DefendTheDisabled@xxxxxxxxx
> >> [2]: https://lore.kernel.org/20260407014129.GC6192@frogsfrogsfrogs
> >>
> >> Assisted-by: Gemini:3.8-flash-high antigravity-cli-1.4.7
> >> Signed-off-by: koraynilay <koray.fra@xxxxxxxxx>
> >> ---
> >>  fs/file_attr.c            | 33 +++++++++++++++++++++++++++++++--
> >>  fs/stat.c                 |  2 ++
> >>  include/linux/fileattr.h  |  5 +++++
> >>  include/linux/stat.h      |  1 +
> >>  include/uapi/linux/fs.h   |  8 +++++++-
> >>  include/uapi/linux/stat.h |  7 ++++++-
> >>  6 files changed, 52 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/fs/file_attr.c b/fs/file_attr.c
> >> index 833e03810b24..c0645cb22bf3 100644
> >> --- a/fs/file_attr.c
> >> +++ b/fs/file_attr.c
> >> @@ -102,7 +102,7 @@ EXPORT_SYMBOL(vfs_fileattr_get);
> >>  static void fileattr_to_file_attr(const struct file_kattr *fa,
> >>                                   struct file_attr *fattr)
> >>  {
> >> -       __u32 mask = FS_XFLAGS_MASK;
> >> +       __u32 mask = FS_FILE_ATTR_XFLAGS_MASK;
> >>
> >>         memset(fattr, 0, sizeof(struct file_attr));
> >>         fattr->fa_xflags = fa->fsx_xflags & mask;
> >> @@ -110,6 +110,8 @@ static void fileattr_to_file_attr(const struct file_kattr *fa,
> >>         fattr->fa_nextents = fa->fsx_nextents;
> >>         fattr->fa_projid = fa->fsx_projid;
> >>         fattr->fa_cowextsize = fa->fsx_cowextsize;
> >> +       fattr->fa_ptime_sec = fa->fsx_ptime.tv_sec;
> >> +       fattr->fa_ptime_nsec = fa->fsx_ptime.tv_nsec;
> >>  }
> >>
> >>  /**
> >> @@ -141,15 +143,20 @@ EXPORT_SYMBOL(copy_fsxattr_to_user);
> >>  static int file_attr_to_fileattr(const struct file_attr *fattr,
> >>                                  struct file_kattr *fa)
> >>  {
> >> -       __u64 mask = FS_XFLAGS_MASK;
> >> +       __u64 mask = FS_FILE_ATTR_XFLAGS_MASK;
> >
> > That is not the correct way to extend this API.
> > This fixed mask was added as a placeholder for
> > "We intend to add support for more xflags to selective filesystems":
> > https://lore.kernel.org/linux-fsdevel/20250329143312.1350603-2-amir73il@xxxxxxxxx/
> >
> > Please see this POC patch for filesystems to publish their supported
> > xflags mask:
> > https://lore.kernel.org/linux-fsdevel/20250329143312.1350603-3-amir73il@xxxxxxxxx/
>
> Thanks, I did not think of adding a dynamic mask like that, I will look
> into it and implement it for v4.
>
> > This will allow callers of file_getattr() to know if the filesystem
> > supports ptime
> > without trying to set it and failing.
>
> So after adding fsx_xflags_mask to file_kattr, every filesystem would
> populate fsx_xflags_mask with all the returned (== supported) FS_XFLAG_*
> in their own specific .fileattr_get, correct?

Not all filesystems. Just the ones that wish to extend the constant
legacy FS_XFLAGS_MASK.
So just the ones that you change to support XFLAGS_PTIME

Thanks,
Amir.