Message-ID: <CAKFNMon_3A7dC+k1q_RjEnoXXNtxBJAUQud_FD4s4VrHZdCVzg@mail.gmail.com>
Date:   Fri, 11 Aug 2023 01:39:10 +0900
From:   Ryusuke Konishi <konishi.ryusuke@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 02/12] nilfs2: use setup_bdev_super to de-duplicate the
 mount code

On Thu, Aug 10, 2023 at 8:05 PM Jan Kara wrote:
>
> 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@....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

One thing I have to tell you as a premise is that nilfs2's snapshot
mounts (cno != 0) are read-only.

The read-only option is mandatory for nilfs2 snapshot mounts, so
remounting to read/write mode will result in an error.
This constraint is checked in nilfs_parse_snapshot_option() which is
called from nilfs_identify().

In fact, any write mode file/inode operations on a snapshot mount will
result in an EROFS error, regardless of whether the coexisting current
tree mount is read-only or read/write (i.e. regardless of the
read-only flag of the superblock instance).

This is mostly (and possibly entirely) accomplished at the vfs layer
by checking the MNT_READONLY flag in mnt_flags of the vfsmount
structure, and even on the nilfs2 side,  iops->permission
(=nilfs_permission) rejects write operations on snapshot mounts.

Therefore, the problem you pointed out shouldn't occur in the first
place since the situation where a snapshot with a handle in write mode
suddenly becomes read-only doesn't happen.   Unless I'm missing
something..

Regards,
Ryusuke Konishi

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