Re: [RFC PATCH] fs: xattr-based FS_IOC_[GS]ETFLAGS interface

From: Theodore Ts'o
Date: Tue Jan 07 2014 - 12:05:12 EST

On Tue, Jan 07, 2014 at 07:49:35AM -0800, Christoph Hellwig wrote:
> On Tue, Jan 07, 2014 at 01:48:31PM +0100, Jan Kara wrote:
> > I have to say I'm not thrilled by the idea of juggling strings in
> > userspace and in kernel to set a flag for an inode...
> Nevermind the massive amounts of code that sit in the filesystem.

The reason for this patch was to address what Dave Chinner has called
"a shitty interface"[1]. Using bitfields that need to be coordinated
across file systems, when sometimes a bit assignment is validly a fs
specific thing, and then later becomes something that gets shared
across file systems.


If we don't go about it this way, there are alternatives: we could
create new ioctls (or a new syscall) as we start running out of bits
used by FS_IOC_[GS]ETFLAGS. We can create new ioctls for bits which
are intended for fs-specific flags, which then later get promoted to
the new syscall when some functionality starts to get shared accross
other file systems (probably with a different bit assignment). This
is certainly less code, but it does mean more complexity outside of
the code when we try to coordinate new functionality across file

Personally, I don't mind dealing with codepoint assignments, but my
impression is that this is a minority viewpoint. Al and Linus have
historically hated bitfields, and Al in the past has spoken favorably
of Plan 9's approach of using strings for the system interface.

So while I have a preference towards using bitfields, as opposed to
using the xattr approach, what I'd really like is that we make a
decision, one way or another, about what's the best way to move

- Ted
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