Re: [PATCH] xfs: add regression test for DAX mount option usage

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Sep 13, 2017 at 3:01 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Wed, Sep 13, 2017 at 08:42:15AM -0600, Ross Zwisler wrote:
>> On Wed, Sep 13, 2017 at 09:47:29AM +1000, Dave Chinner wrote:
>>
>> <>
>>
>> > I think similar concerns exist with using perf, too....
>>
>> I though that using perf addressed both concerns?
>>
>> > > > So what happens when the user is already tracing the test to
>> > > > find a bug and the test turns all their tracing off?
>>
>> By using perf we isolate our tracing from whatever other tracing is happening
>> in the system.  So, unlike the case where we were messing with a system-wide
>> ftrace knob, we run perf on our executable, and someone else can run perf on
>> their executable, and they don't collide.
>
> Yes, you've addressed the "gets inteh way of other tracing concern,
> but it's doesn't address the "it's an ugly way to determine a
> feature is active" concerns. It also creates an implicit stable
> tracepoint out of whatever you are looking at. i.e. that tracepoint
> can't go away, nor can it change functionality once a userspace app
> depends on it's current semantics to function correctly.
>
> And....
>
>> > > > Regardless of this screwing up developer bug triage, do we really
>> > > > want to add a dependency on kernel tracing into the test harness?
>>
>> Yep, you're right that this adds a dependency on perf.  But unfortunately,
>> without using either perf or ftrace, I don't know of a way to detect whether
>> or not DAX is actually being used.  Can you think of another way?
>
> ... if there isn't a programmatic interface to tell applications
> that DAX is in use, then perhaps we're missing a formal userspace
> API that we should have, yes?
>
>> I tried to do this correctly and just skip the test with _notrun
>> if perf isn't available on the host system.  This is the same
>> thing that happens if you are missing other dependencies for a
>> test (some other command (chacl, getfattr, setfattr) not present,
>> quota tools not installed, required users not present, etc).
>
> Sure, but if we have user configurable functionality, then using
> something like ftrace/perf to discover if it's turned is indicative
> of a bigger problem. i.e. that the user can't tell if the
> functionality they turned on/off is actually active or not.
>
> That's a bug that needs fixing, not working around with
> ftrace/perf in xfstests...
>

Yes, we need a way to determine that DAX is enabled, but certainly not
a VM_DAX flag as that was NAKd by you here:

    https://lkml.org/lkml/2016/9/16/13

I think one consideration is to just build this into MAP_SYNC.
MAP_SYNC can only succeed if DAX is enabled and that it is a reliable
way for userspace to assume DAX. There is some consensus for this
proposal here:

    https://lists.01.org/pipermail/linux-nvdimm/2017-August/012086.html