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 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.

> 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.

> 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?
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