[linux-lvm] Re: LVM 2.2 snapshot bug

Rik van Riel riel at conectiva.com.br
Tue Nov 7 14:56:59 UTC 2000

On Tue, 7 Nov 2000, Andrea Arcangeli wrote:
> On Tue, Nov 07, 2000 at 11:55:42AM +0100, Rik van Riel wrote:
> > On snapshot creation, the snapshot block device (/dev/vg0/snap1)
> > is NOT made a read-only device, so ext3 tries to do journal
> It is made read only.

No it isn't. It is made into a read-write block device
where all writes fail :)

kernel: Bad lvm_map in ll_rw_block
kernel: lvm - lvm_map: ll_rw_blk write for readonly LV /dev/vg0/snap1

> > (leading to all kinds of nasty oopses)
> Could you show me the Oopses?

That's a bit much to type in by hand ... and it's basically
kjournald being confused by all its writes failing on a RW
block device.

> > It should be easy enough to do an set_device_ro() on the LVM
> > snapshot, shouldn't it?
> That shouldn't be necessary. The way LVM handle this looks correct to
> me,

Please look again...

> but maybe it's never been tested in the ll_rw_block layer because the
> open(O_RDWR) check always handled it with ext2. Maybe ext3 forces
> writes via ll_rw_block also when the device is mounted read only
> (probably when doing log reply?) and maybe it hits the ll_rw_block
> check for the first time.

Indeed this is the case. When the block device is read-write
(the is_read_only(blk_dev) is non-true) it tries to replay
the log, even for a read-only mounted FS.

> Currently we choose if a device is readable or not using the VM_WRITE
> bitflag in the lv->lv_access field. If the bitflag is set the device
> is writeable. You'll find that the snapshots has that bitflag unset
> (please verify via /proc that they don't have the W capability set).

That bitflag is indeed not set, the snapshot is read-only,
but that status isn't propagated up to the block device

> Or maybe the bug is in ext3 that doesn't handle real read only blockdevices?

Real read only block devices show their status at the block
device layer :)

> Also please ensure you can reproduce with 2.2.18pre17aa1 or
> 2.4.0-test10 to make sure we're looking at the same sources.

It's with the LVM from the Conectiva kernel RPM, which
uses the source code from your 2.2 LVM driver.


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