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
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.
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
additional burden...
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
#ifdef __ppc64__
prctl(MAP_SYNC_ENABLE, 1, 0, 0, 0));
#endif
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...
-aneesh
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