On Fri, May 29, 2020 at 3:55 AM Aneesh Kumar K.V <aneesh.ku...@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.ku...@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.
