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@xxxxxxxxxx>
> > ---
> > 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?
>
> 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 = {
> >
