Re: sget() misuse in nilfs

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, May 06, 2009 at 12:37:29AM +0900, Ryusuke Konishi wrote:
> 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.
> 
>  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.

That's really rather messy...  Let's see if I've got it right:

* r/w -> r/w.  Allowed.
* r/w -> r/o.  Allowed.
* r/w -> snapshot.  Not allowed.
* snapshot -> r/w.  Allowed if it's the latest one and no r/w is there.
* snapshot -> r/o.  It remains a snapshot, but says it has succeeded.
* snapshot -> snapshot.  Only if it's the same.
* r/o -> r/w.  Allowed [1]
* r/o -> r/o.  Allowed.
* r/o -> snapshot.  Allowed only if the snapshot number is the latest.

r/w can't coexist with r/o, but can coexist with any snapshots.  Can't be
remounted to a snapshot directly, but can go through r/w->r/o->latest snapshot
in two mount -o remount.

"r/o" in the above means "read-only, SNAPSHOT flag not set".

What happens if you mount the thing r/w, remount it r/o and then try to
mount the latest snapshot?  Will that give two superblocks or will it
reuse the r/o mount?

OTOH, what will happen if you take r/w mount, mount the latest snapshot and
then remount the r/w one to r/o?

[1] there couldn't have been new r/w mount while r/o one existed, snapshot
number couldn't have changed and the only possible transition *into* r/o is
from r/w, so another r/w superblock couldn't have survived since before our
superblock has become r/o.