Re: [PATCH 1/2] [RFC] vfs: 'stat light' fstatat flags

[Jamie Lokier <jamie@shareable.org>]

Andreas Dilger wrote:
> > > Instead, as was proposed in the last generation of this thread,
> > > there should be
> > > AT_GET_{ATIME,MTIME,CTIME,BLOCKS,SIZE,NLINKS,...}, to return the
> > > flags that the application actually wants.
> > 
> > Well, I respectfully disagree. In my opinion, this adds unncessary
> > complexity to an already targetted case, where only a handful of
> > file systems will even care to optimize away a couple of
> > fields. In that light, I really think we want simplicity here.
> 
> The difficult part is that the application knows which fields it is going
> to use, and the filesystem knows which fields are expensive to fill in,
> and by merging multiple fields together you are deciding arbitrarily that
> you know better than either.  The main users will be common tools like
> "ls --color" and "find", and they know exactly which fields are needed,
> but they need to do a LOT of stats so making it efficient is worthwhile.

I agree with Andreas.  The issue isn't "what is efficient and simple (now)",
but what makes a good future-compatible interface.

> > The good news is that we still have a
> > very good amount of space in the flags field. We could always go back if we
> > _really_ find the need to micro-optimize to the point of per stat field
> > flags. Again though, at this point I don't really think starting out with a
> > slew of flags is a good idea.
> 
> Similar to the helper macros for S_IRWXU = (S_IRUSR|S_IWUSR|S_IXUSR)
> applications won't necessarily need to specify all flags explicitly.

Yes.

If it's intended to re-use the fstatat system call for this, the
conversion from "no stat field flags" to AT_STAT_ALL should be done in
the general fstatat code.

fstatat() is for doing stat() or lstat() relative to an open
directory.  (Anyone else thing the name is stupid - why isn't it
statat like openat, etc.?)

So how do you do a selective fstat() - attributes of an open file?
Don't waste a flag, and just specify AT_SELF as the pathname value to
mean return the attributes of the fd argument itself.  AT_SELF would
be NULL, or perhaps (const char *) -1 to catch application errors.

-- Jamie