Re: Snapshot target and DAX-capable devices

Mike Snitzer
2018-08-30T18:49:11Z

On Thu, Aug 30 2018 at 5:30am -0400,
Jan Kara <jack@suse.cz> wrote:

> On Tue 28-08-18 13:56:30, Mike Snitzer wrote:
> > On Tue, Aug 28 2018 at 3:50am -0400,
> > Jan Kara <jack@suse.cz> 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.
>
> Well, changing device from DAX-capable to DAX-incapable is problematic for
> filesystem on top of it as well. Filesystems simply don't expect this
> feature of a device can change so they would fail in unexpected ways. Also
> PFNs from the pmem (DAX-capable) device that are already mapped to user page
> tables won't magically become unmapped so those processes will still have
> DAX access to those areas of the device.
>
> But, if both original bdev and COW device are DAX-capable, we *should* be
> able to support snapshotting (and refusing mixing of DAX-capable and
> DAX-incapable devices in a snapshot is IMHO not very surprising to users).
> When creating a snapshot of a device, we need to freeze the filesystem
> using it. That will writeprotect all page tables so we are sure we'll get
> page faults (and thus ->direct_access requests from DM POV) for each write
> attempt to any mapping. Then ->direct_access method of snapshot-origin can
> make sure to copy original contents to the COW-device before returning PFN
> from ->direct_access. Similarly ->direct_access of COW-device can provide
> remapped PFN so everything should work seamlessly from user POV.
>
> So something like the above would seem like the best solution from user
> POV. Implementation of the above would not be completely trivial though as
> far as I'm looking into DM code. We'd have to implement ->direct_access
> paths for dm-snap and also I have a vague memory ->direct_access is not
> allowed to sleep these days and DM uses sleeping locks all around... Dan
> should know how big obstacle would it be to reintroduce the sleeping
> possibility (I'm not currently aware of any particular problem with that
> but I'm not paying close attention to those parts of NVDIMM code).

Thanks for these details Jan. Think Dan is on sabbatical so we'll need
Ross to weigh in.

As you point out, how are the upper layers (e.g. filesystems) supposed
to reliably cope with this runtime switch to from DAX to non-DAX access?

It does look like we'll need the more elaborate work you outlined
above. It could be that Mikulas will have interest, DAX expertise and
time to do the work.

Restating the issue: 4.18 commit dbc626597 switched
drivers/md/dm-table.cdevice_supports_dax() to perform a much more
detailed verification of the device's DAX capabilities by calling
bdev_dax_supported() -- which will actually issue read IO via
dax_direct_access() to validate the DAX support. dm-snapshot-origin's
origin_direct_access() returns -EIO. When trying to create a snapshot
of a DAX enabled linear device, this results in the following error:
kernel: device-mapper: ioctl: can't change device type (old=4 vs new=1) after initial table load.

This is because the active DM device's table is being switched from
using the linear target to snapshot-origin. Because the corresponding
DM type switches from DM_TYPE_DAX_BIO_BASED to DM_TYPE_BIO_BASED
(again because bdev_dax_supported()'s call to dm-snapshot-origin's
origin_direct_access() returns -EIO).

In general I _never_ should have taken commit f6e629bd237 ("dm snap: add
fake origin_direct_access"). It gave the elusion that DAX is supported
by dm-snapshot-origin when in reality it simply returns -EIO. Expecting
that this will "just work" because the bio-based path would be used
instead is extremely fragile.

Until we properly add DAX support to dm-snapshot I'm afraid we really do
need to tolerate this "regression". Since reality is the original
support for snapshot of a DAX DM device never worked in a robust way.
I'm running the risk of making peoples' heads explode but I cannot just
drop everything and scramble to implement all the required DAX changes
in dm-snapshot.

Contributions are welcome!

Mike