[PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

[David Howells]

Neil Brown <neilb at suse.de> wrote:

> This justifies for me why a CIFS client would want to extract the
> creation-time from the CIFS protocol, but not why you want to expose it via a
> generic interface.

It would also be easier for NFSD if the creation time was in struct kstat.
It's included as an optional element in NFSv4.  The same goes for the data
version number.  I'm not sure about the inode generation, I suspect that's used
as part of the FH construction.

However, someone was talking about a userspace NFS daemon, and there they may
want all three bits.  Even Samba may want multiple bits.  Calling getxattr
multiple times per file starts to add up, even for internal values.

Consider further: NFS, for example, could be made to retrieve the creation time
from the server.  This can be merged with the attribute fetch done by the
getattr() call, or it could be done separately by getxattr.  Unless it's stored
in RAM, that's one NFS RPC op versus two.  Okay, that's a bit of an artificial
example, but still.

> Given that we have an extensible attribute framework, it seems wrong to be
> adding new attributes to *stat.  If a given filesystem wants to store certain
> attributes more efficiently, then it is welcome to intercept xattr calls and
> store (say) "cifs.birthtime" directly at a known offset in the inode.

It's not attribute storage I'm thinking about, but making attribute retrieval
more efficient.

> The flip-side of extracting these various attributes is setting them.

I acknowledge that if we went down the getxattr() route, then that
automatically makes setxattr() the obvious candidate for setting things.

But think about it another way: what if you want to set several attributes?
You have to make a bunch of setxattr() calls.  But what if it were possible to
do all of chmod, chgrp, chown, truncate, utimes, set_btime, etc. all in one go,
atomically?  We more or less have this internally in the kernel, and it might
stand to be exposed to userspace.

It might, for example, make untarring that little bit more efficient.

> I'm still pondering those extra flags:
>   FS_SPECIAL_FL
>   FS_AUTOMOUNT_FL
>   FS_AUTOMOUNT_ANY_FL
>   FS_REMOTE_FL
>   FS_ENCRYPTED_FL
>   FS_OFFLINE_FL
> 
> They sound like they might be useful, they are not file-metadata (like
> btime) but rather implementation details (like st_blocks).  So it is probably
> sensible to include them as you have done.

I've split these away from ioc flags as ioc flags is very ext2/3/4 centric, and
those filesystems happily create their own ioc flags sets without updating the
master set.

> If a filesystem is mounted on an network-block-device, or a loop-back of a
> file on NFS, is FS_REMOTE_FL set?
> Is ROT13 enough for FS_ENCRYPTED_FL to be set?
> If the NFS server is "not responding, still trying", should FS_OFFLINE_FL get
> set on all files?
> And I cannot even guess at the different between the two FS_AUTOMOUNT flags.
> I'm sure it is something useful, but doco would be good.  Should one of them
> be set on mountpoints that NFSv4 detects from the server?

Yeah.  I have plans to write documentation for it, but I'd like to have a
clearer idea of what the interface might be before doing that.

But to give you an idea of the flags:

 (*) FS_SPECIAL_FL - Kernel API file from a quasi-filesystem such as /proc or
     /sys - the sort of thing you might not want to expose through NFSD.

 (*) FS_AUTOMOUNT_FL - A named automount/referral point.  You attempt to
     transit this directory and the backing fs will mount something over the
     top.

 (*) FS_AUTOMOUNT_ANY_FL - A directory in which you can look up a non-existent
     directory entry, which will cause that dirent to be fabricated and the
     target filesystem be mounted over the top.  Examples include looking up
     arbitrary cell names in /afs, or arbitrary hostnames in autofs or amd
     indirect mount directories.

 (*) FS_REMOTE_FL - A filesystem object that is assumed not to be stored on the
     computer issuing the request.  It would be quite nice to have loopback NFS
     not set the remote flag and to have NBD mounted filesystems to set the
     remote flag, but this can get quite messy with things like overmounts.

     My thought is that this can be used by a GUI to choose its icons for
     files.

 (*) FS_ENCRYPTED_FL - A file that is stored encrypted and that presumably
     needs a key providing to decrypt it.  CIFS has an attribute bit for this
     (ATTR_ENCRYPTED).

 (*) FS_OFFLINE_FL - A file that isn't immediately available, and that requires
     a connection to the data store to be made.  CIFS has an attribute bit for
     this (ATTR_OFFLINE).  AFS has a field in its volume data and an error code
     indicating that a volume is offline and cannot currently be accessed.

     This could be set by network filesystems for which the network or the
     server is absent for example.  Especially if the lightweight stat is
     requested (non-blocking in essence).

> It would probably help to keep that sort of decision process (complete with
> who to blame) documented in the change-log entry, but one never thinks of
> doing that at the time.

There have been a lot of conflicting opinions on this.  I'm not sure rendering
them into a list in the change log would be that useful.

> Providing everybody imposes exactly the same semantics for "creation time"...

We can invent some for Linux.  The time at which an inode is created would seem
to be a sensible course, but with the ability for the creation time to be set
by archiving tools.  Overwriting an existing inode by truncating it and then
writing it should keep the creation time of the inode.

I think this would then be the same behaviour as Windows.

> "well derided" like high-mem and SMP support?  or "real-time" support and
> priority inheritance?
> I guess the deriders are wrong, and will eventually realise that they are
> wrong.  The difficult bit is we cannot know how long it will take them, or
> how much you have to care.

Almost everyone hates the idea of having a stat function with a variable length
buffer.  To quote Linus:

	the "buffer+buflen" thing is still disgusting.

You might be right, though: the deriders might be wrong; it just doesn't help
at this particular point in time.

> (unambiguous documentation!! the rest is just details)

I normally do write documentation.  It's just that I don't want to have to keep
changing the docs as well as constantly rewriting the code.

David