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

[Jeff Layton]

On Fri, 2022-08-05 at 18:06 -0400, Jeff Layton wrote:
> 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.

I started writing some tests for this today, and hit a bit of a chicken-
and-egg problem:

I'd prefer to use xfs_io for easier maintainability, but the STATX_*
constants are defined via UAPI header. Older kernels don't have them and
old xfs_io programs don't understand or report this value.

Should I just write a one-off binary program for xfstests to fetch this
value for now, or are we better off merging the patchset first, and then
fix xfs_io and then write the tests using the updated xfs_io program?

-- 
Jeff Layton <jlayton at kernel.org>