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

[Dan Williams]

On Mon, Oct 8, 2018 at 2:48 PM Alexander Duyck
<alexander.h.duyck@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> >> ---
> >>
> >> 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.