Re: [PATCH] fs: add the ability for statmount() to report the fs_subtype

[Jan Kara <jack@suse.cz>]

On Wed 06-11-24 08:56:15, Jeff Layton wrote:
> On Wed, 2024-11-06 at 14:37 +0100, Jan Kara wrote:
> > On Wed 06-11-24 08:29:19, Jeff Layton wrote:
> > > /proc/self/mountinfo prints out the sb->s_subtype after the type. In
> > > particular, FUSE makes use of this to display the fstype as
> > > fuse.<subtype>.
> > > 
> > > Add STATMOUNT_FS_SUBTYPE and claim one of the __spare2 fields to point
> > > to the offset into the str[] array. The STATMOUNT_FS_SUBTYPE will only
> > > be set in the return mask if there is a subtype associated with the
> > > mount.
> > > 
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > 
> > Looks good to me. I'm just curious: Do you have any particular user that is
> > interested in getting subtype from statmount(2)?
> > 
> > 
> 
> Thanks! Can I count that as a R-b?

Yes. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

> Meta does. They have some logging internally that tracks mounts, and
> knowing what sort of FUSE mounts they have is useful info (when the
> driver bothers to fill out the field anyway).

OK, I thought it's something like that. It would be nice to mention the
usecase in the changelog.

								Honza


> > >  fs/namespace.c             | 20 +++++++++++++++++++-
> > >  include/uapi/linux/mount.h |  5 ++++-
> > >  2 files changed, 23 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/namespace.c b/fs/namespace.c
> > > index ba77ce1c6788dfe461814b5826fcbb3aab68fad4..5f2fb692449a9c0a15b60549fb9f7bedd10f1f3d 100644
> > > --- a/fs/namespace.c
> > > +++ b/fs/namespace.c
> > > @@ -5006,6 +5006,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
> > >  	return 0;
> > >  }
> > >  
> > > +static int statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
> > > +{
> > > +	struct super_block *sb = s->mnt->mnt_sb;
> > > +
> > > +	seq_puts(seq, sb->s_subtype);
> > > +	return 0;
> > > +}
> > > +
> > >  static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
> > >  {
> > >  	s->sm.mask |= STATMOUNT_MNT_NS_ID;
> > > @@ -5064,6 +5072,13 @@ static int statmount_string(struct kstatmount *s, u64 flag)
> > >  		sm->mnt_opts = seq->count;
> > >  		ret = statmount_mnt_opts(s, seq);
> > >  		break;
> > > +	case STATMOUNT_FS_SUBTYPE:
> > > +		/* ignore if no s_subtype */
> > > +		if (!s->mnt->mnt_sb->s_subtype)
> > > +			return 0;
> > > +		sm->fs_subtype = seq->count;
> > > +		ret = statmount_fs_subtype(s, seq);
> > > +		break;
> > >  	default:
> > >  		WARN_ON_ONCE(true);
> > >  		return -EINVAL;
> > > @@ -5203,6 +5218,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
> > >  	if (!err && s->mask & STATMOUNT_MNT_OPTS)
> > >  		err = statmount_string(s, STATMOUNT_MNT_OPTS);
> > >  
> > > +	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
> > > +		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
> > > +
> > >  	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
> > >  		statmount_mnt_ns_id(s, ns);
> > >  
> > > @@ -5224,7 +5242,7 @@ static inline bool retry_statmount(const long ret, size_t *seq_size)
> > >  }
> > >  
> > >  #define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT | \
> > > -			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS)
> > > +			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | STATMOUNT_FS_SUBTYPE)
> > >  
> > >  static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
> > >  			      struct statmount __user *buf, size_t bufsize,
> > > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> > > index 225bc366ffcbf0319929e2f55f1fbea88e4d7b81..fa206fb56b3b25cf80f7d430e1b6bab19c3220e4 100644
> > > --- a/include/uapi/linux/mount.h
> > > +++ b/include/uapi/linux/mount.h
> > > @@ -173,7 +173,9 @@ struct statmount {
> > >  	__u32 mnt_root;		/* [str] Root of mount relative to root of fs */
> > >  	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
> > >  	__u64 mnt_ns_id;	/* ID of the mount namespace */
> > > -	__u64 __spare2[49];
> > > +	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
> > > +	__u32 __spare1[1];
> > > +	__u64 __spare2[48];
> > >  	char str[];		/* Variable size part containing strings */
> > >  };
> > >  
> > > @@ -207,6 +209,7 @@ struct mnt_id_req {
> > >  #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got fs_type */
> > >  #define STATMOUNT_MNT_NS_ID		0x00000040U	/* Want/got mnt_ns_id */
> > >  #define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
> > > +#define STATMOUNT_FS_SUBTYPE		0x00000100U	/* Want/got subtype */
> > >  
> > >  /*
> > >   * Special @mnt_id values that can be passed to listmount
> > > 
> > > ---
> > > base-commit: 26213e1a6caa5a7f508b919059b0122b451f4dfe
> > > change-id: 20241106-statmount-3f91a7ed75fa
> > > 
> > > Best regards,
> > > -- 
> > > Jeff Layton <jlayton@kernel.org>
> > > 
> 
> -- 
> Jeff Layton <jlayton@kernel.org>
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR