On Tue, 5 May 2009 09:18:11 +0100, Al Viro wrote:
> On Tue, May 05, 2009 at 02:11:29AM +0900, Ryusuke Konishi wrote:
> > Hi,
> > On Sun, 3 May 2009 23:51:36 +0100, Al Viro wrote:
> > > OK, I give up; what _is_ get_sb/remount code supposed to implement?
> >
> > Oh, these functions lack spec comments.
> >
> > I first explain some specs. (I think part of them should be added on
> > the code, later)
> >
> > The nilfs_get_sb() allocates a new super_block struct (sb) or assigns
> > an existing sb to the mount point through sget(). For newly allocated
> > sb it calls nilfs_fill_super() for initialization.
> >
> > The following things are supposed here:
>
> > 1) Every rw-mount on a device shares the same sb (as usual).
>
> OK. That's the first kind of sb; no MS_RDONLY, no SNAPSHOT, snapshot_cno is 0.
>
> > 2) Every sb of snapshot is independent with that of rw-mount or other
> > snapshots if their checkpoint numbers differ. On the other hand,
> > two or more snapshots having the same checkpoint number share a sb
> > wherever possible.
>
> Umm... That's what, MS_RDONLY, SNAPSHOT, snapshot_cno > 0?
Yes, exactly.
> > 3) Snapshots are mountable concurrently with a rw-mount, but a
> > ro-mount is not so because the ro-mount cannot follow changes
> > brought by the rw-mount.
>
> And ro-mount would be MS_RDONLY, no SNAPSHOT, snapshot_cno equal to the
> nilfs_last_cno()?
Yes.
> > b) ro->rw remount is possible only if there is no rw-mount on the
> > device and the checkpoint number of the ro-mount is latest.
>
> Er... How could there be an rw-mount while we have ro one? Your
> (3) above would seem to prohibit that situation...
Oh, meaning of the (b) was ambiguous. How about the following one?
b) Remounting an ro-mount to read-only is possible only if the
checkpoint number of the target ro-mount is latest and there is no
existent rw-mount.
> > device and the checkpoint number of the ro-mount is latest.
> > c) Remounting snapshot to different checkpoints or rw-mount is not
> > allowed.
>
> Where is the second part checked in the current code?
Ah, the second part was wrong. The snapshot with latest checkpoint
number can be remounted into an rw-mount. So it should be:
c) Remounting a snapshot to a different checkpoint is not allowed.
Remounting a snapshot to an rw-mount is possible only if the
target snapshot equals to the latest checkpoint.
> > > Can SNAPSHOT even be there unless you have MS_RDONLY?
> >
> > Yes, it can. Nilfs snapshots can exist concurrently with rw-mount.
>
> On the same superblock, that is... And AFAICS the answer's "no, it can't"
> (we can have rw superblock and snapshot superblock at the same time, but
> those will be different instances of struct superblock).
You are right. It's my misunderstanding.
You meant SNAPSHOT for the NILFS_MOUNT_SNAPSHOT flag on
sb->s_mount_opt (it does appear as SNAPSHOT on the code). So, the
answer is "no, it can't".
Regards,
Ryusuke Konishi
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