Snapshot target and DAX-capable devices

Snapshot target and DAX-capable devices

[Jan Kara]

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.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Kani, Toshi]

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.

Thanks,
-Toshi


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Re: Snapshot target and DAX-capable devices

[Jan Kara]

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...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Mike Snitzer]

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.

I'm open to suggestions.

Mike
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Re: Snapshot target and DAX-capable devices

[Kani, Toshi]

On Tue, 2018-08-28 at 13:56 -0400, 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.
> 
> 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

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Re: Snapshot target and DAX-capable devices

[Jan Kara]

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).

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Mike Snitzer]

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
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Re: [dm-devel] Snapshot target and DAX-capable devices

[Jeff Moyer]

Jan Kara <jack@suse.cz> writes:

> 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.

In your example above, if two processes have a file mapped with
MAP_SHARED, and P1 does a store, the new contents will not be reflected
in P2, right?.  This is different from what is expected, and different
from what happens when the page cache is involved.

I think you'd need to unmap all mappings on a CoW, whether triggered by
a store to an existing mapping or a write(2).

-Jeff
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Re: Snapshot target and DAX-capable devices

[Jeff Moyer]

Mike Snitzer <snitzer@redhat.com> writes:

> 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.

Agreed.

-Jeff
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Re: Snapshot target and DAX-capable devices

[Mikulas Patocka]

On Thu, 30 Aug 2018, Mike Snitzer wrote:

> 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

I think a proper fix would be to add functions such as start_dax(struct 
block_device *) and stop_dax(struct block_device *).

start_dax would be used by a (filesystem or other) driver that intends to 
use dax - stop_dax would be used when the driver is being unloaded and it 
no longer needs dax. Device mapper would then maintain a counter how many 
dax users are there and prevent reloading the table if there are any.

Do the persistent memory maintainers intend to add such functions?

Mikulas
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Re: Snapshot target and DAX-capable devices

[Mikulas Patocka]

On Thu, 30 Aug 2018, Jeff Moyer wrote:

> Mike Snitzer <snitzer@redhat.com> writes:
> 
> > 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.
> 
> Agreed.
> 
> -Jeff

You can't support dax on snapshot - if someone maps a block and the block 
needs to be moved, then what?

Mikulas
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Re: Snapshot target and DAX-capable devices

[Jeff Moyer]

Mikulas Patocka <mpatocka@redhat.com> writes:

> On Thu, 30 Aug 2018, Jeff Moyer wrote:
>
>> Mike Snitzer <snitzer@redhat.com> writes:
>> 
>> > 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.
>> 
>> Agreed.
>> 
>> -Jeff
>
> You can't support dax on snapshot - if someone maps a block and the block 
> needs to be moved, then what?

That's exactly the point I brought up in my reply to Jan.  You'd have to
unmap all mappings of the page/block.

-Jeff
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Re: Snapshot target and DAX-capable devices

[Dave Chinner]

On Thu, Aug 30, 2018 at 02:49:07PM -0400, Mike Snitzer wrote:
> On Thu, Aug 30 2018 at  5:30am -0400,  Jan Kara <jack@suse.cz> wrote:
> > 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.
....
> 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?

They can't right now. There's unsolved races between page faults,
invalidations and changing the file operations to/from DAX
dynamically. This is the entire problem facing the dynamic per-inode
DAX on/off flag - if it happens globally to the filesystem without
warning, then the filesystem is screwed. 

To support the block device changing between DAX and non-DAX
dynamically, then the filesystem needs to first invalidate the entire
filesystem cache, eject all cached inodes from memory, any cached
metadata that is using DAX, etc to clear out all the DAX mappings
it have. And it has to do it without racing with new page faults or
IO that might map new DAX pages. And I'm ignoring the fact that we
can't eject referenced inodes (i.e. open files) from the inode cache
and so we currently cannot safely change the DAX on such files.
That's a blocker right now.

Once we can safely change the DAX state of open files, we've got to
co=ordinate the block device state change with the filesystem - the
filesystem wide invalidation has to be done before the block device can
start the change of state, and the filesystem must remain completely
stopped until the block device has completed it's change of state.

So AFAICT this ends up being "stop the world instantly, eject the
world from memory, rebuild the world from scratch, start the world
again".  Freezing the filesystem doesn't stop the world - we can
still do read IO and page faults, so that doesn't prevent pagefault
races with the invalidation leaving DAX references in the page
cache. Hence we currently have no valid "stop the world" mechanism
in the kernel other than unmount, which we can't do while there are
open files.

What about MAP_SYNC applications? If we turn off DAX with those
applications still running, we silently break them and users won't
know until the system loses power and they see data corruption after
the system comes back. However, applications SEGVing unpredictably
becuse of "transparent" storage state changes is almost as
unfriendly.

Dynamically changing block device DAX support seems like a
non-starter to me. At least, it's a non starter until we add a lot
more infrastructure, solve a bunch of really hard problems and
define how active userspace controlled DAX-only features behave when
DAX is no longer available...

Cheers,

Dave.
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david@fromorbit.com
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Re: Snapshot target and DAX-capable devices

[Dave Chinner]

On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote:
> 
> 
> On Thu, 30 Aug 2018, Jeff Moyer wrote:
> 
> > Mike Snitzer <snitzer@redhat.com> writes:
> > 
> > > 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.
> > 
> > Agreed.
> > 
> > -Jeff
> 
> You can't support dax on snapshot - if someone maps a block and the block 
> needs to be moved, then what?

This is only a problem for access via mmap and page faults.

At the filesystem level, it's no different to the existing direct IO
algorithm for read/write IO - we simply allocate new space, copy the
data we need to copy into the new space (may be no copy needed), and
then write the new data into the new space. I'm pretty sure that for
bio-based IO to dm-snapshot devices the algorithm will be exactly
the same.

However, for direct access via mmap, we have to modify how the
userspace virtual address is mapped to the physical location. IOWs,
during the COW operation, we have to invalidate all existing user
mappings we have for that physical address. This means we have to do
an invalidation after the allocate/copy part of the COW operation.

If we are doing this during a page fault, it means we'll probably
have to restart the page fault so it can look up the new physical
address associated with the faulting user address. After we've done
the invalidation, any new (or restarted) page fault finds the
location of new copy we just made, maps it into the user address
space, updates the ptes and we're all good.

Well, that's the theory. We haven't implemented this for XFS yet, so
it might end up a little different, and we might yet hit unexpected
problems (it's DAX, that's what happens :/).

It's a whole different ballgame for a dm-snapshot device - block
devices are completely unaware of page faults to DAX file mappings.
We'll need the filesystem to be aware it's on a remappable block
device, and when we take a DAX write fault we'll need to ask the
underlying device to remap the block and treat it like the
filesystem COW case above. We'll need to do this remap/invalidate
dance in the write IO path, too, because a COW by the block device
is no different to filesystem COW in that path.

Basically, it's the same algorithm as the filesystem COW case, we
just get the physical location of the data and the notification of
block changing physical location from a different interface.

Hmmmm. ISTR that someone has been making a few noises recently about
virtual block address space mapping interfaces that could help solve
this problem....

Cheers,

Dave.
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Re: [dm-devel] Snapshot target and DAX-capable devices

[Jan Kara]

On Thu 30-08-18 15:17:16, Jeff Moyer wrote:
> Jan Kara <jack@suse.cz> writes:
> 
> > 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.
> 
> In your example above, if two processes have a file mapped with
> MAP_SHARED, and P1 does a store, the new contents will not be reflected
> in P2, right?.  This is different from what is expected, and different
> from what happens when the page cache is involved.
> 
> I think you'd need to unmap all mappings on a CoW, whether triggered by
> a store to an existing mapping or a write(2).

Yes, you are right. For COW-device we need to unmap all DAX mappings before
doing CoW. But for snapshot-origin device, we don't need that, right? As
for that case no block actually changes location. So there notification to
DM on first write access should be enough. Am I understanding the problem
right?

								Honza
-- 
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SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Jan Kara]

On Fri 31-08-18 09:38:09, Dave Chinner wrote:
> On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote:
> > 
> > 
> > On Thu, 30 Aug 2018, Jeff Moyer wrote:
> > 
> > > Mike Snitzer <snitzer@redhat.com> writes:
> > > 
> > > > 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.
> > > 
> > > Agreed.
> > > 
> > > -Jeff
> > 
> > You can't support dax on snapshot - if someone maps a block and the block 
> > needs to be moved, then what?
> 
> This is only a problem for access via mmap and page faults.
> 
> At the filesystem level, it's no different to the existing direct IO
> algorithm for read/write IO - we simply allocate new space, copy the
> data we need to copy into the new space (may be no copy needed), and
> then write the new data into the new space. I'm pretty sure that for
> bio-based IO to dm-snapshot devices the algorithm will be exactly
> the same.
> 
> However, for direct access via mmap, we have to modify how the
> userspace virtual address is mapped to the physical location. IOWs,
> during the COW operation, we have to invalidate all existing user
> mappings we have for that physical address. This means we have to do
> an invalidation after the allocate/copy part of the COW operation.
>
> If we are doing this during a page fault, it means we'll probably
> have to restart the page fault so it can look up the new physical
> address associated with the faulting user address. After we've done
> the invalidation, any new (or restarted) page fault finds the
> location of new copy we just made, maps it into the user address
> space, updates the ptes and we're all good.
> 
> Well, that's the theory. We haven't implemented this for XFS yet, so
> it might end up a little different, and we might yet hit unexpected
> problems (it's DAX, that's what happens :/).

Yes, that's outline of a plan :)

> It's a whole different ballgame for a dm-snapshot device - block
> devices are completely unaware of page faults to DAX file mappings.

Actually, block devices are not completely unaware of DAX page faults -
they will get ->direct_access callback for the fault range. It does not
currently convey enough information - we also need to inform the block
device whether it is read or write. But that's about all that's needed to
add AFAICT. And by comparing returned PFN with the one we have stored in
the radix tree (which we have if that file offset is mapped by anybody),
the filesystem / DAX code can tell whether remapping happened and do the
unmapping.

> We'll need the filesystem to be aware it's on a remappable block
> device, and when we take a DAX write fault we'll need to ask the
> underlying device to remap the block and treat it like the
> filesystem COW case above. We'll need to do this remap/invalidate
> dance in the write IO path, too, because a COW by the block device
> is no different to filesystem COW in that path.

Right.

> Basically, it's the same algorithm as the filesystem COW case, we
> just get the physical location of the data and the notification of
> block changing physical location from a different interface.
> 
> Hmmmm. ISTR that someone has been making a few noises recently about
> virtual block address space mapping interfaces that could help solve
> this problem....

:-) Yes, virtual block address space mapping would be a nice solution for
this. But that's a bit larger overhaul, isn't it?

								Honza
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SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Jan Kara]

On Thu 30-08-18 14:49:07, Mike Snitzer wrote:
> 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.

Ross was on vacation as well and I didn't get any email from him for a few
weeks. I'm not sure when he'll be back. So I guess we are on our own.

> 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?

As Dave wrote, switching of underlying device from DAX to non-DAX would be
very difficult to implement currently to that's IMHO a no-go.

> 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.

OK, thanks for including him.

> 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.

Yeah, I don't think the regression is critical. I just wanted to point out
that it exists and that we should look into fixing it...

								Honza

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Re: Snapshot target and DAX-capable devices

[Jan Kara]

On Thu 30-08-18 15:44:57, Mikulas Patocka wrote:
> On Thu, 30 Aug 2018, Mike Snitzer wrote:
> 
> > 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
> 
> I think a proper fix would be to add functions such as start_dax(struct 
> block_device *) and stop_dax(struct block_device *).
> 
> start_dax would be used by a (filesystem or other) driver that intends to 
> use dax - stop_dax would be used when the driver is being unloaded and it 
> no longer needs dax. Device mapper would then maintain a counter how many 
> dax users are there and prevent reloading the table if there are any.
> 
> Do the persistent memory maintainers intend to add such functions?

So that would be a quick way of at least somehow supporting snapshots for
dax-capable devices. Actually these "start_dax / stop_dax" functions
already exist in filesystems - they are fs_dax_get_by_bdev() and
fs_put_dax(). So by plumbing these two calls down into the block layer, you
could easily get the functionality you want. I'm fine with that as a short
term solution for the regression. Longer term I'd like to see something
like we've outlined with Dave to be implemented but obviously that's more
work and also on fs / DAX side, not only DM.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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Re: Snapshot target and DAX-capable devices

[Dave Chinner]

On Fri, Aug 31, 2018 at 11:42:55AM +0200, Jan Kara wrote:
> On Fri 31-08-18 09:38:09, Dave Chinner wrote:
> > On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote:
> > > You can't support dax on snapshot - if someone maps a block and the block 
> > > needs to be moved, then what?
> > 
> > This is only a problem for access via mmap and page faults.
....
> > It's a whole different ballgame for a dm-snapshot device - block
> > devices are completely unaware of page faults to DAX file mappings.
> 
> Actually, block devices are not completely unaware of DAX page faults -
> they will get ->direct_access callback for the fault range. It does not
> currently convey enough information - we also need to inform the block
> device whether it is read or write. But that's about all that's needed to
> add AFAICT. And by comparing returned PFN with the one we have stored in
> the radix tree (which we have if that file offset is mapped by anybody),
> the filesystem / DAX code can tell whether remapping happened and do the
> unmapping.

I forgot about the direct access call.

But it seems like a hack to redefine the simple, fast sector-to-pfn
translation into a slow and potentially resource hungry interface
for physical storage reallocation.  Doing storage layer COW
operations inside direct_access takes us straight back to the bad
ways of get_block() interfaces. We moved all the filesystem
allocation to iomap so that the storage management is separated from
the mm/physical address translation side of DAX - doing block device
storage management operations inside ->direct_access effectively
reverts that separation and so just seems like a hack to me.

Oh, right, DAX. Silly me. :/

Cheers,

Dave.
-- 
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david@fromorbit.com
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Re: Snapshot target and DAX-capable devices

[Huaisheng Ye]

 ---- On Fri, 31 Aug 2018 17:42:55 +0800 Jan Kara <jack@suse.cz> wrote ---- 
 > On Fri 31-08-18 09:38:09, Dave Chinner wrote: 
 > > On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote: 
 > > >  
 > > >  
 > > > On Thu, 30 Aug 2018, Jeff Moyer wrote: 
 > > >  
 > > > > Mike Snitzer <snitzer@redhat.com> writes: 
 > > > >  
 > > > > > 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. 
 > > > >  
 > > > > Agreed. 
 > > > >  
 > > > > -Jeff 
 > > >  
 > > > You can't support dax on snapshot - if someone maps a block and the block  
 > > > needs to be moved, then what? 
 > >  
 > > This is only a problem for access via mmap and page faults. 
 > >  
 > > At the filesystem level, it's no different to the existing direct IO 
 > > algorithm for read/write IO - we simply allocate new space, copy the 
 > > data we need to copy into the new space (may be no copy needed), and 
 > > then write the new data into the new space. I'm pretty sure that for 
 > > bio-based IO to dm-snapshot devices the algorithm will be exactly 
 > > the same. 
 > >  
 > > However, for direct access via mmap, we have to modify how the 
 > > userspace virtual address is mapped to the physical location. IOWs, 
 > > during the COW operation, we have to invalidate all existing user 
 > > mappings we have for that physical address. This means we have to do 
 > > an invalidation after the allocate/copy part of the COW operation. 
 > > 
 > > If we are doing this during a page fault, it means we'll probably 
 > > have to restart the page fault so it can look up the new physical 
 > > address associated with the faulting user address. After we've done 
 > > the invalidation, any new (or restarted) page fault finds the 
 > > location of new copy we just made, maps it into the user address 
 > > space, updates the ptes and we're all good. 
 > >  
 > > Well, that's the theory. We haven't implemented this for XFS yet, so 
 > > it might end up a little different, and we might yet hit unexpected 
 > > problems (it's DAX, that's what happens :/). 
 >  
 > Yes, that's outline of a plan :) 
 >  
 > > It's a whole different ballgame for a dm-snapshot device - block 
 > > devices are completely unaware of page faults to DAX file mappings. 
 >  
 > Actually, block devices are not completely unaware of DAX page faults - 
 > they will get ->direct_access callback for the fault range. It does not 
 > currently convey enough information - we also need to inform the block 
 > device whether it is read or write. But that's about all that's needed to 
 > add AFAICT. And by comparing returned PFN with the one we have stored in 
 > the radix tree (which we have if that file offset is mapped by anybody), 
 > the filesystem / DAX code can tell whether remapping happened and do the 
 > unmapping. 

Hi Jan,

I am trying to investigate how to make dm-snapshot to support DAX, and I
dropped a patchset to upstream for comments. Any suggestion is welcome.
# https://lkml.org/lkml/2018/11/21/281

In the beginning, I haven't considered the situation of mmap write faults.
>From Dan's reply and this email thread, now I have a more clear understanding.

The question is that, even the virtual dm block device has been informed that
the mmap may have write operations through PROT_WRITE, if userspace directly
operate the virtual address of origin device like memcpy, dm-snapshot doesn't
have chance to detect this behavior.
Although dm-snapshot can have chance to prepare a COW area to back up origin's
blocks within ->direct_access callback for the fault range, how can it to have
opportunity to read the data from origin device and save it to COW?

---
Cheers,
Huaisheng Ye

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Re: Snapshot target and DAX-capable devices

[Christoph Hellwig]

Does it really make sense to enhance dm-snapshot?  I thought all serious
users of snapshots had moved on to dm-thinp?
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Re: Snapshot target and DAX-capable devices

[Mike Snitzer]

On Wed, Dec 12 2018 at 11:12am -0500,
Christoph Hellwig <hch@infradead.org> wrote:

> Does it really make sense to enhance dm-snapshot?  I thought all serious
> users of snapshots had moved on to dm-thinp?

There are cases where dm-snapshot is still useful for people.  But those
are very niche users.  I'm not opposed to others proposing enhancements
for dm-snapshot in general but it is definitely not a priority (Google's
dm-bow is an example of a case where dm-snapshot may get extended to
fulfill google's needs).

But for this specific DAX case, I can only assume efforts to prop up
dm-snapshot like this are born out of legacy use-cases.  The reality is
getting DAX to work with dm-snapshot is pretty involved (due to mmap,
etc).  This thread got into a lot of the details:
https://www.redhat.com/archives/dm-devel/2018-August/msg00211.html

So any new attempt to reintroduce DAX support to dm-snapshot (or any
more complex DM target) will have a very high bar.  One that requires
much more extensive mm and block (interlock) prep work to make DAX
safe.

Mike
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Re: Snapshot target and DAX-capable devices

[Kani, Toshi]

On Wed, 2018-12-12 at 12:50 -0500, Mike Snitzer wrote:
> On Wed, Dec 12 2018 at 11:12am -0500,
> Christoph Hellwig <hch@infradead.org> wrote:
> 
> > Does it really make sense to enhance dm-snapshot?  I thought all serious
> > users of snapshots had moved on to dm-thinp?
> 
> There are cases where dm-snapshot is still useful for people.  But those
> are very niche users.  I'm not opposed to others proposing enhancements
> for dm-snapshot in general but it is definitely not a priority (Google's
> dm-bow is an example of a case where dm-snapshot may get extended to
> fulfill google's needs).
> 
> But for this specific DAX case, I can only assume efforts to prop up
> dm-snapshot like this are born out of legacy use-cases.  The reality is
> getting DAX to work with dm-snapshot is pretty involved (due to mmap,
> etc).  This thread got into a lot of the details:
> https://www.redhat.com/archives/dm-devel/2018-August/msg00211.html
> 
> So any new attempt to reintroduce DAX support to dm-snapshot (or any
> more complex DM target) will have a very high bar.  One that requires
> much more extensive mm and block (interlock) prep work to make DAX
> safe.

Just to be clear, "reintroduce DAX support to dm-snapshot" is a bit
misleading because there wasn't such attempt before.  My original
hack/change in dm-snapshot was to support non-DAX use-cases to work with
DAX-capable devices.

Thanks,
-Toshi
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Re: Snapshot target and DAX-capable devices

[Theodore Y. Ts'o]

On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote:
> On Wed, Dec 12 2018 at 11:12am -0500,
> Christoph Hellwig <hch@infradead.org> wrote:
> 
> > Does it really make sense to enhance dm-snapshot?  I thought all serious
> > users of snapshots had moved on to dm-thinp?
> 
> There are cases where dm-snapshot is still useful for people.  But those
> are very niche users.  I'm not opposed to others proposing enhancements
> for dm-snapshot in general but it is definitely not a priority (Google's
> dm-bow is an example of a case where dm-snapshot may get extended to
> fulfill google's needs).

I would expect that dm-snapshot will be used quite a lot for
short-lived snapshots (that only live during a database backup or an
fsck run).  I would hardly call that a "niche use case".  One other
major advantage that dm-snapshot has is that you can take a snapshot
for any LVM volume.  For dm-thinp you have migrate your storage to a
thinp pool, and that adds a fair amount of friction to users migrating
to dm-thinp.

						- Ted

Re: Snapshot target and DAX-capable devices

[Mike Snitzer]

On Wed, Dec 12 2018 at  4:15pm -0500,
Theodore Y. Ts'o <tytso@mit.edu> wrote:

> On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote:
> > On Wed, Dec 12 2018 at 11:12am -0500,
> > Christoph Hellwig <hch@infradead.org> wrote:
> > 
> > > Does it really make sense to enhance dm-snapshot?  I thought all serious
> > > users of snapshots had moved on to dm-thinp?
> > 
> > There are cases where dm-snapshot is still useful for people.  But those
> > are very niche users.  I'm not opposed to others proposing enhancements
> > for dm-snapshot in general but it is definitely not a priority (Google's
> > dm-bow is an example of a case where dm-snapshot may get extended to
> > fulfill google's needs).
> 
> I would expect that dm-snapshot will be used quite a lot for
> short-lived snapshots (that only live during a database backup or an
> fsck run).  I would hardly call that a "niche use case".

dm-snapshot is only ~60% performant for 1 snapshot.  Try to do
additional snapshots and performance crawls to a stop (though I haven't
reassessed performance in a while).

dm-snapshot has been in Linux since before 2005, I don't know of all the
users of it -- maybe there are a ton of users who only take a single
temporary snapshot and we're all oblivious.

Definitely not seeing many bugs against it (but it has been around
forever).  I do know that there are relatively few people showing
interest in it.  But for 4.21 I did stage a couple useful performance
fixes:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=61d594bb7e1cf86dca49cbc9524eb80169d9fca6
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.21&id=d1f7898c7a1b24aa9ae670f9cc21b65e730827eb

> One other major advantage that dm-snapshot has is that you can take a
> snapshot for any LVM volume.  For dm-thinp you have migrate your
> storage to a thinp pool, and that adds a fair amount of friction to
> users migrating to dm-thinp.

dm-thinp has the concept of an "external origin".  Changes to origin LVM
volume get copied out to the thin-pool (same copy cost as old
dm-snapshot).  But IIRC from that point on your LVM volume is a dm-thin
device.

Mike
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Re: [dm-devel] Snapshot target and DAX-capable devices

[Theodore Y. Ts'o]

On Wed, Dec 12, 2018 at 05:43:22PM -0500, Mike Snitzer wrote:
> > I would expect that dm-snapshot will be used quite a lot for
> > short-lived snapshots (that only live during a database backup or an
> > fsck run).  I would hardly call that a "niche use case".
> 
> dm-snapshot is only ~60% performant for 1 snapshot.  Try to do
> additional snapshots and performance crawls to a stop (though I haven't
> reassessed performance in a while).
> 
> dm-snapshot has been in Linux since before 2005, I don't know of all the
> users of it -- maybe there are a ton of users who only take a single
> temporary snapshot and we're all oblivious.

Well, here's one user:

https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/scrub/e2scrub.in

This is a spiffed up version of my original script:

https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/contrib/e2croncheck

I suppose we should look into enhancing e2scrub so it can deal with
volumes stored on dm-thin pools.....

> dm-thinp has the concept of an "external origin".  Changes to origin LVM
> volume get copied out to the thin-pool (same copy cost as old
> dm-snapshot).  But IIRC from that point on your LVM volume is a dm-thin
> device.

I would think the *snapshot* would have to be the dm-thin device, not
the origin volume, correct?

The original LVM module would still be mounted through the original
LVM device-mapper device, so it couldn't get transmogrified to be a
dm-thin device, right?

					- Ted
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RE: [External] Re: Snapshot target and DAX-capable devices

[Huaisheng HS1 Ye]

From: Mike Snitzer <snitzer@redhat.com>
Sent: Thursday, December 13, 2018 6:43 AM
> On Wed, Dec 12 2018 at  4:15pm -0500,
> Theodore Y. Ts'o <tytso@mit.edu> wrote:
> 
> > On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote:
> > > On Wed, Dec 12 2018 at 11:12am -0500,
> > > Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > > > Does it really make sense to enhance dm-snapshot?  I thought all serious
> > > > users of snapshots had moved on to dm-thinp?
> > >
> > > There are cases where dm-snapshot is still useful for people.  But those
> > > are very niche users.  I'm not opposed to others proposing enhancements
> > > for dm-snapshot in general but it is definitely not a priority (Google's
> > > dm-bow is an example of a case where dm-snapshot may get extended to
> > > fulfill google's needs).
> >
> > I would expect that dm-snapshot will be used quite a lot for
> > short-lived snapshots (that only live during a database backup or an
> > fsck run).  I would hardly call that a "niche use case".
> 
> dm-snapshot is only ~60% performant for 1 snapshot.  Try to do
> additional snapshots and performance crawls to a stop (though I haven't
> reassessed performance in a while).
> 
> dm-snapshot has been in Linux since before 2005, I don't know of all the
> users of it -- maybe there are a ton of users who only take a single
> temporary snapshot and we're all oblivious.
> 
> Definitely not seeing many bugs against it (but it has been around
> forever).  I do know that there are relatively few people showing
> interest in it.  But for 4.21 I did stage a couple useful performance
> fixes:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.
> 21&id=61d594bb7e1cf86dca49cbc9524eb80169d9fca6
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.
> 21&id=d1f7898c7a1b24aa9ae670f9cc21b65e730827eb

Hi Mike,

Could these two patches be applied to current code of LVM?
Although there is a difficult problem as mmap for dm-snapshot with
DAX-capable, the two patches can be used for other complex DM targets
when trying to implement DAX.

[RFC PATCH v2 2/3] dm: expand hc_map in mapped_device for lack of map
https://lkml.org/lkml/2018/11/21/273

[RFC PATCH v2 3/3] dm: expand valid types for dm-ioctl
https://lkml.org/lkml/2018/11/21/276

Cheers,
Huaisheng Ye
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Re: Snapshot target and DAX-capable devices

[Mike Snitzer]

On Fri, Dec 14 2018 at  3:24am -0500,
Huaisheng HS1 Ye <yehs1@lenovo.com> wrote:

> From: Mike Snitzer <snitzer@redhat.com>
> Sent: Thursday, December 13, 2018 6:43 AM
> > On Wed, Dec 12 2018 at  4:15pm -0500,
> > Theodore Y. Ts'o <tytso@mit.edu> wrote:
> > 
> > > On Wed, Dec 12, 2018 at 12:50:47PM -0500, Mike Snitzer wrote:
> > > > On Wed, Dec 12 2018 at 11:12am -0500,
> > > > Christoph Hellwig <hch@infradead.org> wrote:
> > > >
> > > > > Does it really make sense to enhance dm-snapshot?  I thought all serious
> > > > > users of snapshots had moved on to dm-thinp?
> > > >
> > > > There are cases where dm-snapshot is still useful for people.  But those
> > > > are very niche users.  I'm not opposed to others proposing enhancements
> > > > for dm-snapshot in general but it is definitely not a priority (Google's
> > > > dm-bow is an example of a case where dm-snapshot may get extended to
> > > > fulfill google's needs).
> > >
> > > I would expect that dm-snapshot will be used quite a lot for
> > > short-lived snapshots (that only live during a database backup or an
> > > fsck run).  I would hardly call that a "niche use case".
> > 
> > dm-snapshot is only ~60% performant for 1 snapshot.  Try to do
> > additional snapshots and performance crawls to a stop (though I haven't
> > reassessed performance in a while).
> > 
> > dm-snapshot has been in Linux since before 2005, I don't know of all the
> > users of it -- maybe there are a ton of users who only take a single
> > temporary snapshot and we're all oblivious.
> > 
> > Definitely not seeing many bugs against it (but it has been around
> > forever).  I do know that there are relatively few people showing
> > interest in it.  But for 4.21 I did stage a couple useful performance
> > fixes:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.
> > 21&id=61d594bb7e1cf86dca49cbc9524eb80169d9fca6
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.
> > 21&id=d1f7898c7a1b24aa9ae670f9cc21b65e730827eb
> 
> Hi Mike,
> 
> Could these two patches be applied to current code of LVM?
> Although there is a difficult problem as mmap for dm-snapshot with
> DAX-capable, the two patches can be used for other complex DM targets
> when trying to implement DAX.
> 
> [RFC PATCH v2 2/3] dm: expand hc_map in mapped_device for lack of map
> https://lkml.org/lkml/2018/11/21/273
> 
> [RFC PATCH v2 3/3] dm: expand valid types for dm-ioctl
> https://lkml.org/lkml/2018/11/21/276

No I'm not taking these patches.  They are hacks that allow DM targets
to do things that aren't supportable (yet).
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