From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:45976 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726959AbeH3Xst (ORCPT ); Thu, 30 Aug 2018 19:48:49 -0400 Date: Thu, 30 Aug 2018 15:44:57 -0400 (EDT) From: Mikulas Patocka To: Mike Snitzer cc: Jan Kara , Jeff Moyer , "Kani, Toshi" , "linux-nvdimm@lists.01.org" , "dm-devel@redhat.com" , "linux-fsdevel@vger.kernel.org" , "ross.zwisler@linux.intel.com" , "dan.j.williams@intel.com" Subject: Re: Snapshot target and DAX-capable devices In-Reply-To: <20180830184907.GA14867@redhat.com> Message-ID: References: <20180827160744.GE4002@quack2.suse.cz> <20180828075025.GA17756@quack2.suse.cz> <20180828175630.GA1197@redhat.com> <20180830093028.GC1767@quack2.suse.cz> <20180830184907.GA14867@redhat.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, 30 Aug 2018, Mike Snitzer wrote: > On Thu, Aug 30 2018 at 5:30am -0400, > Jan Kara wrote: > > > On Tue 28-08-18 13:56:30, Mike Snitzer wrote: > > > On Tue, Aug 28 2018 at 3:50am -0400, > > > Jan Kara 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