Re: [PATCH v2 11/15] xfs: mount api - add xfs_reconfigure()

[Brian Foster <bfoster@redhat.com>]

On Fri, Aug 30, 2019 at 07:10:44PM +0800, Ian Kent wrote:
> On Wed, 2019-08-28 at 09:28 -0400, Brian Foster wrote:
> > On Fri, Aug 23, 2019 at 09:00:16AM +0800, Ian Kent wrote:
> > > Add the fs_context_operations method .reconfigure that performs
> > > remount validation as previously done by the super_operations
> > > .remount_fs method.
> > > 
> > > An attempt has also been made to update the comment about options
> > > handling problems with mount(8) to reflect the current situation.
> > > 
> > > Signed-off-by: Ian Kent <raven@themaw.net>
> > > ---
> > >  fs/xfs/xfs_super.c |   84
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 84 insertions(+)
> > > 
> > > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > > index 76374d602257..aae0098fecab 100644
> > > --- a/fs/xfs/xfs_super.c
> > > +++ b/fs/xfs/xfs_super.c
> > > @@ -1522,6 +1522,89 @@ xfs_fs_remount(
> > >  	return 0;
> > >  }
> > >  
> > > +/*
> > > + * There have been problems in the past with options passed from
> > > mount(8).
> > > + *
> > > + * The problem being that options passed by mount(8) in the case
> > > where only
> > > + * the the mount point path is given would consist of the existing
> > > fstab
> > > + * options with the options from mtab for the current mount merged
> > > in and
> > > + * the options given on the command line last. But the result
> > > couldn't be
> > > + * relied upon to accurately reflect the current mount options so
> > > that
> > > + * rejecting options that can't be changed on reconfigure could
> > > erronously
> > > + * cause mount failure.
> > > + *
> > > + * The mount-api uses a legacy mount options handler in the VFS to
> > > handle
> > > + * mount(8) so these options will continue to be passed. Even if
> > > mount(8)
> > > + * is updated to use fsopen()/fsconfig()/fsmount() it's likely to
> > > continue
> > > + * to set the existing options so options problems with
> > > reconfigure could
> > > + * continue.
> > > + *
> > > + * For the longest time mtab locking was a problem and this could
> > > have been
> > > + * one possible cause. It's also possible there could have been
> > > options
> > > + * order problems.
> > > + *
> > > + * That has changed now as mtab is a link to the proc file system
> > > mount
> > > + * table so mtab options should be always accurate.
> > > + *
> > > + * Consulting the util-linux maintainer (Karel Zak) he is
> > > confident that,
> > > + * in this case, the options passed by mount(8) will be those of
> > > the current
> > > + * mount and the options order should be a correct merge of fstab
> > > and mtab
> > > + * options, and new options given on the command line.
> > > + *
> > > + * So, in theory, it should be possible to compare incoming
> > > options and
> > > + * return an error for options that differ from the current mount
> > > and can't
> > > + * be changed on reconfigure to prevent users from believing they
> > > might have
> > > + * changed mount options using remount which can't be changed.
> > > + *
> > > + * But for now continue to return success for every reconfigure
> > > request, and
> > > + * silently ignore all options that can't actually be changed.
> > > + */
> > 
> > This seems like all good information for a commit log description or
> > perhaps to land in a common header where some of these fs context
> > bits
> > are declared, but overly broad for a function header comment for an
> > XFS
> > callback. I'd more expect some information around the fundamental
> > difference between 'mp' and 'new_mp,' where those come from, what
> > they
> > mean, etc. From the code, it seems like new_mp is transient and
> > reflects
> > changes that we need to incorporate in the original mp..?
> 
> The reason I looked into this is becuase of a fairly lengthy
> comment in the original source and that lead to this rather
> verbose tail about what I found.
> 
> It's not so much a mount context problem becuase it's due
> to the way user space constructs the mount options from
> various sources where options are present.
> 
> So I don't think it's something that the mount context
> code can resolve.
> 
> Which leaves a bunch of (I think) useful information
> without a palce to live so it can be referred to ...
> suggestions?
> 

I suppose there's always the commit log description if there isn't a
suitable place in the code.

Brian

> > 
> > Brian
> > 
> > > +STATIC int
> > > +xfs_reconfigure(
> > > +	struct fs_context *fc)
> > > +{
> > > +	struct xfs_fs_context	*ctx = fc->fs_private;
> > > +	struct xfs_mount	*mp = XFS_M(fc->root->d_sb);
> > > +	struct xfs_mount        *new_mp = fc->s_fs_info;
> > > +	xfs_sb_t		*sbp = &mp->m_sb;
> > > +	int			flags = fc->sb_flags;
> > > +	int			error;
> > > +
> > > +	error = xfs_validate_params(new_mp, ctx, false);
> > > +	if (error)
> > > +		return error;
> > > +
> > > +	/* inode32 -> inode64 */
> > > +	if ((mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
> > > +	    !(new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
> > > +		mp->m_flags &= ~XFS_MOUNT_SMALL_INUMS;
> > > +		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp-
> > > >sb_agcount);
> > > +	}
> > > +
> > > +	/* inode64 -> inode32 */
> > > +	if (!(mp->m_flags & XFS_MOUNT_SMALL_INUMS) &&
> > > +	    (new_mp->m_flags & XFS_MOUNT_SMALL_INUMS)) {
> > > +		mp->m_flags |= XFS_MOUNT_SMALL_INUMS;
> > > +		mp->m_maxagi = xfs_set_inode_alloc(mp, sbp-
> > > >sb_agcount);
> > > +	}
> > > +
> > > +	/* ro -> rw */
> > > +	if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(flags & SB_RDONLY)) {
> > > +		error = xfs_remount_rw(mp);
> > > +		if (error)
> > > +			return error;
> > > +	}
> > > +
> > > +	/* rw -> ro */
> > > +	if (!(mp->m_flags & XFS_MOUNT_RDONLY) && (flags & SB_RDONLY)) {
> > > +		error = xfs_remount_ro(mp);
> > > +		if (error)
> > > +			return error;
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > > +
> > >  /*
> > >   * Second stage of a freeze. The data is already frozen so we only
> > >   * need to take care of the metadata. Once that's done sync the
> > > superblock
> > > @@ -2049,6 +2132,7 @@ static const struct super_operations
> > > xfs_super_operations = {
> > >  static const struct fs_context_operations xfs_context_ops = {
> > >  	.parse_param = xfs_parse_param,
> > >  	.get_tree    = xfs_get_tree,
> > > +	.reconfigure = xfs_reconfigure,
> > >  };
> > >  
> > >  static struct file_system_type xfs_fs_type = {
> > > 
>