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

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

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.

Actually I think you are probably right. Do you want to get that or 
should I. Should be a quick patch since you could probably just add a 
call to percpu_ref_get_many to hold a reference for each page in the 
range of device pages before calling memmap_init_zone_device.

- Alex