Re: [PATCH 02/12] nilfs2: use setup_bdev_super to de-duplicate the mount code

Jan Kara
2023-08-10T11:05:53Z

On Fri 04-08-23 11:01:39, Ryusuke Konishi wrote:
> On Thu, Aug 3, 2023 at 8:46 PM Jan Kara wrote:
> >
> > On Wed 02-08-23 17:41:21, Christoph Hellwig wrote:
> > > Use the generic setup_bdev_super helper to open the main block device
> > > and do various bits of superblock setup instead of duplicating the
> > > logic. This includes moving to the new scheme implemented in common
> > > code that only opens the block device after the superblock has allocated.
> > >
> > > It does not yet convert nilfs2 to the new mount API, but doing so will
> > > become a bit simpler after this first step.
> > >
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> >
> > AFAICS nilfs2 could *almost* use mount_bdev() directly and then just do its
>
> > snapshot thing after mount_bdev() returns. But it has this weird logic
> > that: "if the superblock is already mounted but we can shrink the whole
> > dcache, then do remount instead of ignoring mount options". Firstly, this
> > looks racy - what prevents someone from say opening a file on the sb just
> > after nilfs_tree_is_busy() shrinks dcache? Secondly, it is inconsistent
> > with any other filesystem so it's going to surprise sysadmins not
> > intimately knowing nilfs2. Thirdly, from userspace you cannot tell what
> > your mount call is going to do. Last but not least, what is it really good
> > for? Ryusuke, can you explain please?
> >
> > Honza
>
> I think you are referring to the following part:
>
> > if (!s->s_root) {
> ...
> > } else if (!sd.cno) {
> > if (nilfs_tree_is_busy(s->s_root)) {
> > if ((flags ^ s->s_flags) & SB_RDONLY) {
> > nilfs_err(s,
> > "the device already has a %s mount.",
> > sb_rdonly(s) ? "read-only" : "read/write");
> > err = -EBUSY;
> > goto failed_super;
> > }
> > } else {
> > /*
> > * Try remount to setup mount states if the current
> > * tree is not mounted and only snapshots use this sb.
> > */
> > err = nilfs_remount(s, &flags, data);
> > if (err)
> > goto failed_super;
> > }
> > }
>
> What this logic is trying to do is, if there is already a nilfs2 mount
> instance for the device, and are trying to mounting the current tree
> (sd.cno is 0, so this is not a snapshot mount), then will switch
> depending on whether the current tree has a mount:
>
> - If the current tree is mounted, it's just like a normal filesystem.
> (A read-only mount and a read/write mount can't coexist, so check
> that, and reuse the instance if possible)
> - Otherwise, i.e. for snapshot mounts only, do whatever is necessary
> to add a new current mount, such as starting a log writer.
> Since it does the same thing that nilfs_remount does, so
> nilfs_remount() is used there.
>
> Whether or not there is a current tree mount can be determined by
> d_count(s->s_root) > 1 as nilfs_tree_is_busy() does.
> Where s->s_root is always the root dentry of the current tree, not
> that of the mounted snapshot.

I see now, thanks for explanation! But one thing still is not clear to me.
If you say have a snapshot mounted read-write and then you mount the
current snapshot (cno == 0) read-only, you'll switch the whole superblock
to read-only state. So also the mounted snapshot is suddently read-only
which is unexpected and actually supposedly breaks things because you can
still have file handles open for writing on the snapshot etc.. So how do
you solve that?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR