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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-08-16 at 15:44 +0200, Christian Brauner wrote:
> On Tue, Aug 16, 2022 at 09:27:56AM -0400, Jeff Layton wrote:
> > From: Jeff Layton <jlayton@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 display the change attribute and
> > the mountid as well.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  fs/stat.c                 | 7 +++++++
> >  include/linux/stat.h      | 1 +
> >  include/uapi/linux/stat.h | 3 ++-
> >  samples/vfs/test-statx.c  | 8 ++++++--
> >  4 files changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/stat.c b/fs/stat.c
> > index 9ced8860e0f3..7c3d063c31ba 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_CHANGE_ATTR) && IS_I_VERSION(inode)) {
> > +		stat->result_mask |= STATX_CHANGE_ATTR;
> > +		stat->change_attr = inode_query_iversion(inode);
> > +	}
> > +
> >  	mnt_userns = mnt_user_ns(path->mnt);
> >  	if (inode->i_op->getattr)
> >  		return inode->i_op->getattr(mnt_userns, path, stat,
> > @@ -611,6 +617,7 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
> >  	tmp.stx_dev_major = MAJOR(stat->dev);
> >  	tmp.stx_dev_minor = MINOR(stat->dev);
> >  	tmp.stx_mnt_id = stat->mnt_id;
> > +	tmp.stx_change_attr = stat->change_attr;
> >  
> >  	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
> >  }
> > diff --git a/include/linux/stat.h b/include/linux/stat.h
> > index 7df06931f25d..7b444c2ad0ad 100644
> > --- a/include/linux/stat.h
> > +++ b/include/linux/stat.h
> > @@ -50,6 +50,7 @@ struct kstat {
> >  	struct timespec64 btime;			/* File creation time */
> >  	u64		blocks;
> >  	u64		mnt_id;
> > +	u64		change_attr;
> >  };
> >  
> >  #endif
> > diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> > index 1500a0f58041..fd839ec76aa4 100644
> > --- a/include/uapi/linux/stat.h
> > +++ b/include/uapi/linux/stat.h
> > @@ -124,7 +124,7 @@ struct statx {
> >  	__u32	stx_dev_minor;
> >  	/* 0x90 */
> >  	__u64	stx_mnt_id;
> > -	__u64	__spare2;
> > +	__u64	stx_change_attr; /* Inode change attribute */
> >  	/* 0xa0 */
> >  	__u64	__spare3[12];	/* Spare space for future expansion */
> >  	/* 0x100 */
> > @@ -152,6 +152,7 @@ struct statx {
> >  #define STATX_BASIC_STATS	0x000007ffU	/* The stuff in the normal stat struct */
> >  #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
> >  #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
> > +#define STATX_CHANGE_ATTR	0x00002000U	/* Want/got stx_change_attr */
> 
> I'm a bit worried that STATX_CHANGE_ATTR isn't a good name for the flag
> and field. Or I fail to understand what exact information this will
> expose and how userspace will consume it.
> To me the naming gives the impression that some set of generic
> attributes have changed but given that statx is about querying file
> attributes this becomes confusing.
> 
> Wouldn't it make more sense this time to expose it as what it is and
> call this STATX_INO_VERSION and __u64 stx_ino_version?

Ok, having thought about this some more, I think this is a reasonable
name. It _does_ sort of imply that this value will increase over time.
That's true of all of the existing implementations, but I think we ought
to define such that there could be alternative implementations.

I'll respin this patch and resend it with a wider audience.

Thanks for the input so far!
-- 
Jeff Layton <jlayton@kernel.org>