On 6/1/20 3:39 PM, Jan Kara wrote:
On Fri 29-05-20 16:25:35, Aneesh Kumar K.V wrote:
> On 5/29/20 3:22 PM, Jan Kara wrote:
>> 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
>>> 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
>>>>> The kernel should prevent the usage of MAP_SYNC if applications are
>>>>> the new instructions on newer hardware.
>>>>> This patch adds a prctl option MAP_SYNC_ENABLE that can be used to
>>>>> the usage of MAP_SYNC. The kernel config option is added to allow the
>>>>> to control whether MAP_SYNC should be enabled by default or not.
>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar(a)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
>>>>> static unsigned long default_dump_filter =
>>>>> +#ifdef CONFIG_ARCH_MAP_SYNC_DISABLE
>>>>> +unsigned long default_map_sync_mask = MMF_DISABLE_MAP_SYNC_MASK;
>>>>> +unsigned long default_map_sync_mask = 0;
>> I'm not sure CONFIG is really the right approach here. For a distro that
>> 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.
Hum, couldn't we set some flag for nvdimm devices with
"ibm,persistent-memory-v2" property and then check it during mmap(2) time
and when the device has this propery and the mmap(2) caller doesn't have
the prctl set, we'd disallow MAP_SYNC? That should make things mostly
seamless, shouldn't it? Only apps that want to use MAP_SYNC on these
devices would need to use prctl(MMF_DISABLE_MAP_SYNC, 0) but then these
applications need to be aware of new instructions so this isn't that much
I am not sure application would want to add that much details/knowledge
about a platform in their code. I was expecting application to do
prctl(MAP_SYNC_ENABLE, 1, 0, 0, 0));
a = mmap(NULL, PAGE_SIZE, PROT_READ|PROT_WRITE,
MAP_SHARED_VALIDATE | MAP_SYNC, fd, 0);
For that code all the complexity that we add w.r.t
ibm,persistent-memory-v2 is not useful. Do you see a value in making all
these device specific rather than a conditional on __ppc64__?
> 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?
This is a bit of a gamble... I don't have too much trust in certification /
verification because only the "big players" may do powerfail testing
throughout enough that they'd uncover these problems. So the question
really is: How many apps are out there using MAP_SYNC on ppc64? Hopefully
not many given the HW didn't ship yet as you wrote but I have no real clue.
Similarly there's a question: How many app writers will read manual for
older ppc64 architecture and write apps that won't work reliably on
POWER10? Again, I have no idea.
So the prctl would be IMHO a nice safety belt but I'm not 100% certain it
will be needed...