Re: [RFC PATCH 1/2] libnvdimm: Add prctl control for disabling synchronous fault support.

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

On Fri, May 29, 2020 at 3:55 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 5/29/20 3:22 PM, Jan Kara wrote:
> > Hi!
> >
> > On Fri 29-05-20 15:07:31, Aneesh Kumar K.V wrote:
> >> Thanks Michal. I also missed Jeff in this email thread.
> >
> > And I think you'll also need some of the sched maintainers for the prctl
> > bits...
> >
> >> On 5/29/20 3:03 PM, Michal Suchánek wrote:
> >>> Adding Jan
> >>>
> >>> On Fri, May 29, 2020 at 11:11:39AM +0530, Aneesh Kumar K.V wrote:
> >>>> With POWER10, architecture is adding new pmem flush and sync instructions.
> >>>> The kernel should prevent the usage of MAP_SYNC if applications are not using
> >>>> the new instructions on newer hardware.
> >>>>
> >>>> This patch adds a prctl option MAP_SYNC_ENABLE that can be used to enable
> >>>> the usage of MAP_SYNC. The kernel config option is added to allow the user
> >>>> to control whether MAP_SYNC should be enabled by default or not.
> >>>>
> >>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> > ...
> >>>> diff --git a/kernel/fork.c b/kernel/fork.c
> >>>> index 8c700f881d92..d5a9a363e81e 100644
> >>>> --- a/kernel/fork.c
> >>>> +++ b/kernel/fork.c
> >>>> @@ -963,6 +963,12 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(mmlist_lock);
> >>>>    static unsigned long default_dump_filter = MMF_DUMP_FILTER_DEFAULT;
> >>>> +#ifdef CONFIG_ARCH_MAP_SYNC_DISABLE
> >>>> +unsigned long default_map_sync_mask = MMF_DISABLE_MAP_SYNC_MASK;
> >>>> +#else
> >>>> +unsigned long default_map_sync_mask = 0;
> >>>> +#endif
> >>>> +
> >
> > I'm not sure CONFIG is really the right approach here. For a distro that would
> > basically mean to disable MAP_SYNC for all PPC kernels unless application
> > explicitly uses the right prctl. Shouldn't we rather initialize
> > default_map_sync_mask on boot based on whether the CPU we run on requires
> > new flush instructions or not? Otherwise the patch looks sensible.
> >
>
> yes that is correct. We ideally want to deny MAP_SYNC only w.r.t
> POWER10. But on a virtualized platform there is no easy way to detect
> that. We could ideally hook this into the nvdimm driver where we look at
> the new compat string ibm,persistent-memory-v2 and then disable MAP_SYNC
> if we find a device with the specific value.
>
> BTW with the recent changes I posted for the nvdimm driver, older kernel
> won't initialize persistent memory device on newer hardware. Newer
> hardware will present the device to OS with a different device tree
> compat string.
>
> My expectation  w.r.t this patch was, Distro would want to  mark
> CONFIG_ARCH_MAP_SYNC_DISABLE=n based on the different application
> certification.  Otherwise application will have to end up calling the
> prctl(MMF_DISABLE_MAP_SYNC, 0) any way. If that is the case, should this
> be dependent on P10?
>
> With that I am wondering should we even have this patch? Can we expect
> userspace get updated to use new instruction?.
>
> With ppc64 we never had a real persistent memory device available for
> end user to try. The available persistent memory stack was using vPMEM
> which was presented as a volatile memory region for which there is no
> need to use any of the flush instructions. We could safely assume that
> as we get applications certified/verified for working with pmem device
> on ppc64, they would all be using the new instructions?

I think prctl is the wrong interface for this. I was thinking a sysfs
interface along the same lines as /sys/block/pmemX/dax/write_cache.
That attribute is toggling DAXDEV_WRITE_CACHE for the determination of
whether the platform or the kernel needs to handle cache flushing
relative to power loss. A similar attribute can be established for
DAXDEV_SYNC, it would simply default to off based on a configuration
time policy, but be dynamically changeable at runtime via sysfs.

These flags are device properties that affect the kernel and
userspace's handling of persistence.