Re: [RFC PATCH v2 1/2] vfs: add provenance time (ptime)

[koraynilay]

On Wed Oct 28, 2026 at 3:56 PM CET, Pedro Falcato wrote:
> On Wed, Oct 28, 2026 at 02:47:27PM +0100, koraynilay 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.
>
> Random comment, but: could this be doable with xattrs instead? I'm not sure
> where I would put the xattrs code (if in userspace exclusively, or if hooking
> the VFS up with this, or if doing it on the filesystem side), but it would
> surely be less disruptive than modifying a bunch of filesystems one-by-one.
>

Thanks for the comment, I actually wanted to include the "why not xattr"
explanation in the cover letter but totally forgot lol.

I actually have been using fully userspace xattrs[1] for a few months,
mostly for backups, and it "works"; but last week I wanted to merge 2
FSs (and have another merge I have to do), so I ran [1] on all files and
then compared all metadata/data between source and destination and
noticed there were quite a lot of files that didn't have the xattr set.
So I looked online and learned that xattrs only work on regular
files/directories, not on symlinks or anything else (and the missing
ones were of symlinks).
This is what prompted me to search more if anyone had already proposed a
new timestamp (instead of making btime settable) and found the linked
email thread from april, which gave me the excuse to finally work on a
proper patch.

[1]: https://github.com/koraynilay/btime-to-xattr

> This patch is quite simple, but changing btrfs, then ext4, xfs, etc is a
> mess, and inevitably you can't get these timestamps until you upgrade your
> filesystems.

So yeah while I do agree it touches a lot of code that could create
problems, IMO this is the best solution and it would complete files'
timestamps like other OSs have (and this, I think, is necessary for
both backup/archival and migrations).

Best,
koraynilay

Attachment: signature.asc
Description: PGP signature