Re: [RFC][PATCH 0/4] Enhanced file stat system call

From: David Howells
Date: Thu Nov 17 2016 - 12:03:51 EST

One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> wrote:

> > (2) Lightweight stat (AT_STATX_DONT_SYNC): Ask for just those details of
> > interest, and allow a network fs to approximate anything not of
> > interest, without going to the server.
> >
> > (3) Heavyweight stat (AT_STATX_FORCE_SYNC): Force a network fs to flush
> > buffers and go to the server, even if it thinks its cached attributes
> > are up to date.
> That seems an odd way to do it. Wouldn't it be cleaner and more flexible
> to give a timestamp of the oldest time you consider acceptable (and
> obviously passing 0 indicates whatever you have)

Perhaps, though adding 6-argument syscalls is apparently frowned upon.

> > Note that no lstat() equivalent is required as that can be implemented
> > through statx() with atflag == 0. There is also no fstat() equivalent as
> > that can be implemented through statx() with filename == NULL and the
> > relevant fd passed as dfd.
> and dfd + a name gives you fstatat() ?


> The cover note could be clearer on this.


> Should the fields really be split the way they are for times rather than
> a struct for each one so you can write code generically to handle one of
> those rather than having to have a 4 way switch statement all the time.

It depends. Doing so leaves 16 bytes of hole in the structure. I could
ameliorate the wastage by using a union to overlay useful fields in the gaps,
but that's pretty icky and might be compiler dependent.

> Another attribute that would be nice (but migt need some trivial device
> layer tweaking) would be STATX_ATTR_VOLATILE for filesystems that will
> probably evaporate on a reboot. That's useful information for tools like
> installers and also for sanity checking things like backup paths.

There's a FILE_ATTRIBUTE_TEMPORARY that I could map for windows filesystems
that could be used with this.

> Remote needs to have clear semantics: is ext4fs over nbd 'remote' for
> example ?

Hmmm... Interesting question. Probably should. But you could be insane and
RAID an nbd and a local disk. Further, does NFS over a loopback device to
nfsd on the same machine qualify as root? What if that's exposing a local fs
on NBD? Perhaps I should drop 'REMOTE' for now. It sounds like something
that a GUI filemanager might find interesting, though.