Re: [PATCH 1/6] statx: Add a system call to make enhanced file info available

[Christoph Hellwig]

On Tue, May 10, 2016 at 09:43:43AM +0100, David Howells wrote:
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 
> > All of these are easily available.  But why special case them so that
> > userspace must not ask for them?  This makes an otherwise totally
> > regular interface special now.  Note that filesystems could always fill
> > it out anyway and set it in the return mask.
> 
> Because it would be a waste of bits in the mask.  Is there a point in having
> bits that are always going to be set unconditionally when we can just
> *document* that these few fields are always going to be set.

And what exaxtly is the cost of these bits?

> So yes, you can look on it as there are special cases.  However, if I can drop
> stat emulation support, everything resolves down to the following classes:
> 
>  (1) Stuff that's unconditional: st_dev, st_blksize, st_information (maybe).
> 
>  (2) st_mode & S_IFMT.  Unconditional or conditional?  I'm not sure.
> 
>  (3) Stuff that's conditional: st_mode & ~S_IFMT, st_rdev, st_ino, ...
>      Basically everything else.

If we at least go down to one set of conditional and one optional that's
at least much better than what we currently have.