Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

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

On Mon, Oct 8, 2018 at 3:37 PM Alexander Duyck
<alexander.h.duyck@linux.intel.com> wrote:
>
>
>
> On 10/8/2018 3:00 PM, Dan Williams wrote:
> > On Mon, Oct 8, 2018 at 2:48 PM Alexander Duyck
> > <alexander.h.duyck@linux.intel.com> wrote:
> >>
> >> On 10/8/2018 2:01 PM, Dan Williams wrote:
> >>> On Tue, Sep 25, 2018 at 1:29 PM Alexander Duyck
> >>> <alexander.h.duyck@linux.intel.com> wrote:
> >>>>
> >>>> The ZONE_DEVICE pages were being initialized in two locations. One was with
> >>>> the memory_hotplug lock held and another was outside of that lock. The
> >>>> problem with this is that it was nearly doubling the memory initialization
> >>>> time. Instead of doing this twice, once while holding a global lock and
> >>>> once without, I am opting to defer the initialization to the one outside of
> >>>> the lock. This allows us to avoid serializing the overhead for memory init
> >>>> and we can instead focus on per-node init times.
> >>>>
> >>>> One issue I encountered is that devm_memremap_pages and
> >>>> hmm_devmmem_pages_create were initializing only the pgmap field the same
> >>>> way. One wasn't initializing hmm_data, and the other was initializing it to
> >>>> a poison value. Since this is something that is exposed to the driver in
> >>>> the case of hmm I am opting for a third option and just initializing
> >>>> hmm_data to 0 since this is going to be exposed to unknown third party
> >>>> drivers.
> >>>>
> >>>> Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
> >>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>>> ---
> >>>>
> >>>> v4: Moved moved memmap_init_zone_device to below memmmap_init_zone to avoid
> >>>>       merge conflicts with other changes in the kernel.
> >>>> v5: No change
> >>>
> >>> This patch appears to cause a regression in the "create.sh" unit test
> >>> in the ndctl test suite.
> >>
> >> So all you had to do is run the create.sh script to see the issue? I
> >> just want to confirm there isn't any additional information needed
> >> before I try chasing this down.
> >
> >  From the ndctl source tree run:
> >
> >      make -j TESTS="create.sh" check
> >
> > ...the readme has some more setup instructions:
> > https://github.com/pmem/ndctl/blob/master/README.md
> >
> > 0day has sometimes run this test suite automatically, but we need to
> > get that more robust because setting up this environment is a bit of a
> > hoop to jump through with the need to setup the nfit_test module.
> >
> >>> I tried to reproduce on -next with:
> >>>
> >>> 2302f5ee215e mm: defer ZONE_DEVICE page initialization to the point
> >>> where we init pgmap
> >>>
> >>> ...but -next does not even boot for me at that commit.
> >>
> >> What version of -next? There are a couple of patches probably needed
> >> depending on which version you are trying to boot.
> >
> > Today's -next, but backed up to that above commit. I was also seeing
> > CONFIG_DEBUG_LIST spamming the logs, and a crash in the crypto layer.
> >
> >>> Here is a warning signature that proceeds a hang with this patch
> >>> applied against v4.19-rc6:
> >>>
> >>> percpu ref (blk_queue_usage_counter_release) <= 0 (-1530626) after
> >>> switching to atomic
> >>> WARNING: CPU: 24 PID: 7346 at lib/percpu-refcount.c:155
> >>> percpu_ref_switch_to_atomic_rcu+0x1f7/0x200
> >>> CPU: 24 PID: 7346 Comm: modprobe Tainted: G           OE     4.19.0-rc6+ #2458
> >>> [..]
> >>> RIP: 0010:percpu_ref_switch_to_atomic_rcu+0x1f7/0x200
> >>> [..]
> >>> Call Trace:
> >>>    <IRQ>
> >>>    ? percpu_ref_reinit+0x140/0x140
> >>>    rcu_process_callbacks+0x273/0x880
> >>>    __do_softirq+0xd2/0x428
> >>>    irq_exit+0xf6/0x100
> >>>    smp_apic_timer_interrupt+0xa2/0x220
> >>>    apic_timer_interrupt+0xf/0x20
> >>>    </IRQ>
> >>> RIP: 0010:lock_acquire+0xb8/0x1a0
> >>> [..]
> >>>    ? __put_page+0x55/0x150
> >>>    ? __put_page+0x55/0x150
> >>>    __put_page+0x83/0x150
> >>>    ? __put_page+0x55/0x150
> >>>    devm_memremap_pages_release+0x194/0x250
> >>>    release_nodes+0x17c/0x2c0
> >>>    device_release_driver_internal+0x1a2/0x250
> >>>    driver_detach+0x3a/0x70
> >>>    bus_remove_driver+0x58/0xd0
> >>>    __x64_sys_delete_module+0x13f/0x200
> >>>    ? trace_hardirqs_off_thunk+0x1a/0x1c
> >>>    do_syscall_64+0x60/0x210
> >>>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >>>
> >>
> >> So it looks like we are tearing down memory when this is triggered. Do
> >> we know if this is at the end of the test or if this is running in
> >> parallel with anything?
> >
> > Should not be running in parallel with anything this test is
> > performing a series of namespace setup and teardown events.
> >
> > Wait, where did the call to "percpu_ref_get()" go? I think that's the bug.
> >
>
> I have a reproduction on my system now as well. I should have a patch
> ready to go for it in the next hour or so.
>

Nice! Thanks for jumping on this, and I like the "get_many" optimization.