[RFC PATCH 1/4] vfs: report change attribute in statx for IS_I_VERSION inodes

[Jeff Layton]

On Sat, 2022-08-06 at 08:01 +1000, Dave Chinner wrote:
> On Fri, Aug 05, 2022 at 02:35:40PM -0400, Jeff Layton wrote:
> > From: Jeff Layton <jlayton at redhat.com>
> > 
> > Claim one of the spare fields in struct statx to hold a 64-bit change
> > attribute. When statx requests this attribute, do an
> > inode_query_iversion and fill the result in the field.
> > 
> > Also update the test-statx.c program to fetch the change attribute as
> > well.
> > 
> > Signed-off-by: Jeff Layton <jlayton at kernel.org>
> > ---
> >  fs/stat.c                 | 7 +++++++
> >  include/linux/stat.h      | 1 +
> >  include/uapi/linux/stat.h | 3 ++-
> >  samples/vfs/test-statx.c  | 4 +++-
> >  4 files changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/stat.c b/fs/stat.c
> > index 9ced8860e0f3..976e0a59ab23 100644
> > --- a/fs/stat.c
> > +++ b/fs/stat.c
> > @@ -17,6 +17,7 @@
> >  #include <linux/syscalls.h>
> >  #include <linux/pagemap.h>
> >  #include <linux/compat.h>
> > +#include <linux/iversion.h>
> >  
> >  #include <linux/uaccess.h>
> >  #include <asm/unistd.h>
> > @@ -118,6 +119,11 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> >  	stat->attributes_mask |= (STATX_ATTR_AUTOMOUNT |
> >  				  STATX_ATTR_DAX);
> >  
> > +	if ((request_mask & STATX_CHGATTR) && IS_I_VERSION(inode)) {
> > +		stat->result_mask |= STATX_CHGATTR;
> > +		stat->chgattr = inode_query_iversion(inode);
> > +	}
> 
> If you're going to add generic support for it, shouldn't there be a
> generic test in fstests that ensures that filesystems that advertise
> STATX_CHGATTR support actually behave correctly? Including across
> mounts, and most importantly, that it is made properly stable by
> fsync?
> 
> i.e. what good is this if different filesystems have random quirks
> that mean it can't be relied on by userspace to tell it changes have
> occurred?

Absolutely. Being able to better test the i_version field for consistent
behavior is a primary goal. I haven't yet written any yet, but we'd
definitely want something in xfstests if we decide this is worthwhile.
-- 
Jeff Layton <jlayton at redhat.com>