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

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Date:   Thu, 17 Nov 2016 16:45:45 +0000
From:   David Howells <dhowells@...hat.com>
To:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/4] Enhanced file stat system call

One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> 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() ?

Yes.

> The cover note could be clearer on this.

Fixed.

> 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.

David