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

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Message-ID: <20160512091249.GB22420@infradead.org>
Date:	Thu, 12 May 2016 02:12:49 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org,
	linux-nfs@...r.kernel.org, samba-technical@...ts.samba.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/6] statx: Add a system call to make enhanced file info
 available

On Tue, May 10, 2016 at 09:43:43AM +0100, David Howells wrote:
> Christoph Hellwig <hch@...radead.org> 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.