Re: [PATCH v2 2/7] dax: change bdev_dax_supported() to support boolean returns

[Dan Williams]

On Thu, May 31, 2018 at 7:24 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Thu, May 31, 2018 at 06:57:33PM -0700, Dan Williams wrote:
>> On Thu, May 31, 2018 at 6:26 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> > On Thu, May 31, 2018 at 12:13:32PM -0700, Darrick J. Wong wrote:
>> >> On Tue, May 29, 2018 at 04:01:14PM -0600, Ross Zwisler wrote:
>> >> > On Tue, May 29, 2018 at 02:25:10PM -0700, Darrick J. Wong wrote:
>> >> > > On Tue, May 29, 2018 at 01:51:01PM -0600, Ross Zwisler wrote:
>> >> > > > From: Dave Jiang <dave.jiang@xxxxxxxxx>
>> >> > > >
>> >> > > > The function return values are confusing with the way the function is
>> >> > > > named. We expect a true or false return value but it actually returns
>> >> > > > 0/-errno.  This makes the code very confusing. Changing the return values
>> >> > > > to return a bool where if DAX is supported then return true and no DAX
>> >> > > > support returns false.
>> >> > > >
>> >> > > > Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
>> >> > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> >> > >
>> >> > > Looks ok, do you want me to pull the first two patches through the xfs
>> >> > > tree?
>> >> > >
>> >> > > Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>> >> >
>> >> > Thanks for the review.
>> >> >
>> >> > I'm not sure what's best.  If you do that then Mike will need to have a DM
>> >> > branch for the rest of the series based on your stable commits, yea?
>> >> >
>> >> > Mike what would you prefer?
>> >>
>> >> I /was/ about to say that I would pull in the first two patches, but now
>> >> I can't get xfs to mount with pmem at all, and have no way of testing
>> >> this...?
>> >
>> > I have similar problems, too, but:
>> >
>> > $ ndctl list
>> > [
>> >   {
>> >     "dev":"namespace1.0",
>> >     "mode":"raw",
>> >     "size":8589934592,
>> >     "sector_size":512,
>> >     "blockdev":"pmem1"
>> >   },
>> >   {
>> >     "dev":"namespace0.0",
>> >     "mode":"raw",
>> >     "size":8589934592,
>> >     "sector_size":512,
>> >     "blockdev":"pmem0"
>> >   }
>> > ]
>> > $ sudo ndctl create-namespace -f -e namespace0.0 --mode=fsdax
>> >   Error: operation failed, region0 fsdax mode not available
>> >
>> > failed to reconfigure namespace: Invalid argument
>> > $
>> >
>> > I can't make head or tail of what is going wrong here - how am I
>> > supposed to debug this and get it working again?
>> >
>> > FWIW, XFS+DAX used to just work on this setup (I hadn't even
>> > installed ndctl until this morning!) but after changing the kernel
>> > it no longer works. That would make it a regression, yes?
>>
>> This commit caused the behavior change:
>>
>>     569d0365f571 dax: require 'struct page' by default for filesystem dax
>>
>> The justification is in that patch, but the short summary is we killed
>> off "pageless" dax because it had so many incomplete holes and
>> surprise behaviors. It needed to die on the path to making dax not
>> experimental, i.e. to close safety holes, and be feature complete for
>> all the ways userspace expects to use mappings (direct-io, fork,
>> poison handling, etc).
>>
>> I suspect your kernel does not have CONFIG_ZONE_DEVICE enabled which
>> has the following dependencies:
>>
>>         depends on MEMORY_HOTPLUG
>>         depends on MEMORY_HOTREMOVE
>>         depends on SPARSEMEM_VMEMMAP
>
> Filesystem DAX now has a dependency on memory hotplug?
>
> Fmeh. No wonder I never enabled CONFIG_ZONE_DEVICE. It's described in
> menuconfig as "Device memory (pmem, HMM, etc...) hotplug support".
> This isn't for hotplug support - it required for basic DAX
> functionality. I've never enabled memory hotplug in any of my test
> kernel configs, because the VMs I run the kernels on don't ever get
> memory hotplugged....
>
> OK, works now I've found the magic config incantantions to turn
> everything I now need on.
>
> Can we get this rationalised to a single top level config
> option in the filesystems menu "Enable Filesystem DAX" that turns
> on every knob that is required?

That sounds good to me. Will do.