Re: Bug/regression: Read-only mount not read-only

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Nov 30, 2015 at 11:48:01AM -0500, Chris Mason wrote:
> On Sat, Nov 28, 2015 at 01:46:34PM +0000, Hugo Mills wrote:
> >    We've just had someone on IRC with a problem mounting their FS. The
> > main problem is that they've got a corrupt log tree. That isn't the
> > subject of this email, though.
> > 
> >    The issue I'd like to raise is that even with -oro as a point
> > option, the FS is trying to replay the log tree. The dmesg output from
> > mount -oro is at the end of the email.
> > 
> >    Now, my memory, experience and understanding is that the FS
> > doesn't, and shouldn't replay the log tree on a RO mount, because the
> > FS should still be consistent even without the reply, and
> > RO-means-actually-RO is possible and desirable. (Compared to a
> > journalling FS, where journal replay is required for a consistent,
> > usable FS).
> > 
> >    So, this looks to me like a regression that's come in somewhere.
> > 
> >    (Just for completeness, the system in question usually runs 4.2.5,
> > but the live CD the OP is using is 4.2.3).
> 
> We do need to replay the log tree, even on readonly mounts.  Otherwise
> files created and fsunk before crashing may not even exist.

   I'm actually happy with that, as long as the log tree is retained
until it _can_ be played back. I think it's much more important that
read-only actually means read-only *as much as is possible* (if for no
other reason than being able to test the status of the log tree).
Obviously, for journalling FSes, a journal reply is required by the
design of the FS, but with a CoW FS, the FS should be consistent if
possibly outdated with a RO mount.

   Maybe there should be a "replay-log" mount option to modify the
"ro" option to allow the log to be replayed but no further
modifications? (i.e. keep the plain "ro" case to be the safest option
that makes the fewest changes to the FS structure -- none).

> We'll bail out of the log replay on readonly media, but otherwise the
> replay always happens.

   OK, so what was happening in the cases where a filesystem was
mountable RO, but not RW, and then btrfs-zero-log allowed the FS to be
mounted? I've handled any number of people with exactly those
symptoms, and it's been like that for a while. What I saw on IRC a
couple of days ago seems to be new behaviour.

   Hugo.

-- 
Hugo Mills             | argc, argv, argh!
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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