On Tue, 2018-08-28 at 13:56 -0400, Mike Snitzer wrote: > On Tue, Aug 28 2018 at 3:50am -0400, > Jan Kara <jack@xxxxxxx> wrote: > > > On Mon 27-08-18 16:43:28, Kani, Toshi wrote: > > > On Mon, 2018-08-27 at 18:07 +0200, Jan Kara wrote: > > > > Hi, > > > > > > > > I've been analyzing why fstest generic/081 fails when the backing device is > > > > capable of DAX. The problem boils down to the failure of: > > > > > > > > lvm vgcreate -f vg0 /dev/pmem0 > > > > lvm lvcreate -L 128M -n lv0 vg0 > > > > lvm lvcreate -s -L 4M -n snap0 vg0/lv0 > > > > > > > > The last command fails like: > > > > > > > > device-mapper: reload ioctl on (253:0) failed: Invalid argument > > > > Failed to lock logical volume vg0/lv0. > > > > Aborting. Manual intervention required. > > > > > > > > And the core of the problem is that volume vg0/lv0 is originally of > > > > DM_TYPE_DAX_BIO_BASED type but when the snapshot gets created, we try to > > > > switch it to DM_TYPE_BIO_BASED because now the device stops supporting DAX. > > > > The problem seems to be introduced by Ross' commit dbc626597 "dm: prevent > > > > DAX mounts if not supported". > > > > > > > > The question is whether / how this should be fixed. The current inability > > > > to create snapshots of DAX-capable devices looks weird and the cryptic > > > > failure makes it even worse (it took me quite a while to understand what is > > > > failing and why). OTOH I see the rationale behind Ross' change as well. > > > > > > Here are the dm-snap changes that went along with the original DAX > > > support. > > > > > > commit b5ab4a9ba55 > > > commit f6e629bd237 > > > > > > Basically, snapshots can be added/removed to DAX-capable devices, but > > > snapshots need to be mounted without dax option. > > > > Yes, and after these two commits things were working. But then commit > > dbc626597 broke things again so currently snapshotting DAX-capable devices > > does not work. Just try with 4.18... > > Commit f6e629bd237 was a nasty hack, and commit dbc626597 exposed it as > such. But commit dbc626597 has caused us to regress.. so we need to fix > it. > > We could remove DM_TYPE_DAX_BIO_BASED completely. But in the past I was > reluctant to do so because it really is unclear how/if we can even > support a device switching from DAX to non-DAX while IO is in-flight. DM > supports suspending without flushing (via dmsetup suspend --noflush) and > that could really be problematic if we leave DAX IO inflight and then > switch the DM table such that the DM device no longer supports DAX. > > I'm open to suggestions. Right, commit f6e629bd237 is a hack, but I do not have a better idea at this point... For now, I am afraid that reverting commit dbc626597 may be an option. Thanks, -Toshi -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
