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

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 5/30/20 12:52 AM, Dan Williams wrote:
> 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.
> 

That will not handle the scenario with multiple applications using the 
same fsdax mount point where one is updated to use the new instruction 
and the other is not.

-aneeseh
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