Re: [PATCH] add a FMODE flag to make XFS invisible I/O less hacky

[Christoph Hellwig <hch@lst.de>]

On Fri, Nov 28, 2008 at 03:58:42PM -0700, Andreas Dilger wrote:
> On Nov 28, 2008  12:24 +0100, Christoph Hellwig wrote:
> > XFS has a mode called invisble I/O that doesn't update any of the
> > timestamps.  It's used for HSM-style applications and exposed through
> > the nasty open by handle ioctl.
> > 
> > Instead of doing directly assignment of file operations that set an
> > internal flag for it add a new FMODE_INVISIBLE flag that we can check
> > in the normal file operations.
> 
> Why not call this "FMODE_NOCMTIME" similar to the inode flag "S_NOCMTIME"
> that already exists, and .  That makes it more clear what is being done,
> instead of calling it "INVISIBLE".

The flag is called invisible inside XFS because it also has the
implication of not sending HSM notification when you have dmapi enabled.
Doesn't matter too much on mainline, but for now I'd keep the semantics
the same as the old one, at least as long as it's not exposed to
userland (which Al's suggestions of doing it in the O_ namespace would)

> It should also not be possible to skip ctime updates for non-root users,
> as that provides some forensic trail if files have been modified by users.
> Not sure if that is relevant here though (it looks like this is only used
> internally), but worth mentioning in any case.

Yes, the open by handle ioctl which sets this flag requires CAP_SYS_ADMIN.