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Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

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  Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-04 14:30 +0100
    Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-01-04 15:00 +0100
      Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-04 15:10 +0100
        Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-05 13:10 +0100
          Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-06 13:20 +0100
            Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-01-06 13:30 +0100

#1550770 — Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

On Wed, Dec 14, 2016 at 09:11:47AM +0000, Ard Biesheuvel wrote:
> The NUMA code may get confused by the presence of NOMAP regions within
> zones, resulting in spurious BUG() checks where the node id deviates
> from the containing zone's node id.
> 
> Since the kernel has no business reasoning about node ids of pages it
> does not own in the first place, enable CONFIG_HOLES_IN_ZONE to ensure
> that such pages are disregarded.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm64/Kconfig | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 111742126897..0472afe64d55 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -614,6 +614,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
>  	def_bool y
>  	depends on NUMA
>  
> +config HOLES_IN_ZONE
> +	def_bool y
> +	depends on NUMA
> +
>  source kernel/Kconfig.preempt
>  source kernel/Kconfig.hz

I'm happy to apply this, but I'll hold off until the first patch is queued
somewhere, since this doesn't help without the VM_BUG_ON being moved.

Alternatively, I can queue both if somebody from the mm camp acks the
first patch.

Will

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#1550830

On 4 January 2017 at 13:28, Will Deacon <will.deacon@arm.com> wrote:
> On Wed, Dec 14, 2016 at 09:11:47AM +0000, Ard Biesheuvel wrote:
>> The NUMA code may get confused by the presence of NOMAP regions within
>> zones, resulting in spurious BUG() checks where the node id deviates
>> from the containing zone's node id.
>>
>> Since the kernel has no business reasoning about node ids of pages it
>> does not own in the first place, enable CONFIG_HOLES_IN_ZONE to ensure
>> that such pages are disregarded.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  arch/arm64/Kconfig | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 111742126897..0472afe64d55 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -614,6 +614,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
>>       def_bool y
>>       depends on NUMA
>>
>> +config HOLES_IN_ZONE
>> +     def_bool y
>> +     depends on NUMA
>> +
>>  source kernel/Kconfig.preempt
>>  source kernel/Kconfig.hz
>
> I'm happy to apply this, but I'll hold off until the first patch is queued
> somewhere, since this doesn't help without the VM_BUG_ON being moved.
>
> Alternatively, I can queue both if somebody from the mm camp acks the
> first patch.
>

Actually, I am not convinced the discussion is finalized. These
patches do fix the issue, but Robert also suggested an alternative fix
which may be preferable.

http://marc.info/?l=linux-arm-kernel&m=148190753510107&w=2

I haven't responded to it yet, due to the holidays, but I'd like to
explore that solution a bit further before applying anything, if you
don't mind.

Thanks,
Ard.

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#1550841

On Wed, Jan 04, 2017 at 01:50:20PM +0000, Ard Biesheuvel wrote:
> On 4 January 2017 at 13:28, Will Deacon <will.deacon@arm.com> wrote:
> > On Wed, Dec 14, 2016 at 09:11:47AM +0000, Ard Biesheuvel wrote:
> >> The NUMA code may get confused by the presence of NOMAP regions within
> >> zones, resulting in spurious BUG() checks where the node id deviates
> >> from the containing zone's node id.
> >>
> >> Since the kernel has no business reasoning about node ids of pages it
> >> does not own in the first place, enable CONFIG_HOLES_IN_ZONE to ensure
> >> that such pages are disregarded.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> >> ---
> >>  arch/arm64/Kconfig | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >> index 111742126897..0472afe64d55 100644
> >> --- a/arch/arm64/Kconfig
> >> +++ b/arch/arm64/Kconfig
> >> @@ -614,6 +614,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
> >>       def_bool y
> >>       depends on NUMA
> >>
> >> +config HOLES_IN_ZONE
> >> +     def_bool y
> >> +     depends on NUMA
> >> +
> >>  source kernel/Kconfig.preempt
> >>  source kernel/Kconfig.hz
> >
> > I'm happy to apply this, but I'll hold off until the first patch is queued
> > somewhere, since this doesn't help without the VM_BUG_ON being moved.
> >
> > Alternatively, I can queue both if somebody from the mm camp acks the
> > first patch.
> >
> 
> Actually, I am not convinced the discussion is finalized. These
> patches do fix the issue, but Robert also suggested an alternative fix
> which may be preferable.
> 
> http://marc.info/?l=linux-arm-kernel&m=148190753510107&w=2
> 
> I haven't responded to it yet, due to the holidays, but I'd like to
> explore that solution a bit further before applying anything, if you
> don't mind.

Using early_pfn_valid feels like a bodge to me, since having pfn_valid
return false for something that early_pfn_valid says is valid (and is
therefore initialised in the memmap) makes the NOMAP semantics even more
confusing.

But there's no rush, so I'll hold off for the moment. I was under the
impression that things had stalled.

Will

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#1551936

On Thu, Jan 05, 2017 at 12:24:07PM +0100, Robert Richter wrote:
> On 04.01.17 14:02:23, Will Deacon wrote:
> > Using early_pfn_valid feels like a bodge to me, since having pfn_valid
> > return false for something that early_pfn_valid says is valid (and is
> > therefore initialised in the memmap) makes the NOMAP semantics even more
> > confusing.
> 
> The concern I have had with HOLES_IN_ZONE is that it enables
> pfn_valid_within() for arm64. This means that each pfn of a section is
> checked which is done only once for the section otherwise. With up to
> 2^18 pages per section we traverse the memblock list by that factor
> more often. There could be a performance regression.

There could be, but we're trying to fix a bug here. I wouldn't have
thought that walking over pfns like that is done very often.

> I haven't numbers yet, since the fix causes another kernel crash. And,
> this is the next problem I have. The crash doesn't happen otherwise. So,
> either it uncovers another bug or the fix is incomplete. Though the
> changes look like it should work. This needs more investigation.

I really can't see how the fix causes a crash, and I couldn't reproduce
it on any of my boards, nor could any of the Linaro folk afaik. Are you
definitely running mainline with just these two patches from Ard?

Will

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#1552731

On Thu, Jan 05, 2017 at 08:49:44PM +0100, Robert Richter wrote:
> On 05.01.17 13:22:00, Robert Richter wrote:
> > On 05.01.17 12:08:20, Will Deacon wrote:
> > > I really can't see how the fix causes a crash, and I couldn't reproduce
> > > it on any of my boards, nor could any of the Linaro folk afaik. Are you
> > > definitely running mainline with just these two patches from Ard?
> > 
> > Yes, just both patches applied. Various other solutions were working.
> 
> I have retested the same kernel (v4.9 based) as before and now it
> boots fine including rtc-efi device registration (it was crashing
> there):
> 
>  rtc-efi rtc-efi: rtc core: registered rtc-efi as rtc0
> 
> There could be a difference in firmware and mem setup, though I also
> downgraded the firmware to test it, but can't reproduce it anymore. I
> could reliable trigger the crash the first time.
> 
> FTR the oops.

Hmm, I just can't help but think you were accidentally running with
additional patches when you saw this oops previously. For example,
your log looks very similar to this one:

  http://lists.infradead.org/pipermail/linux-arm-kernel/2016-December/473666.html

but then again, these crashes probably often look alike.

Will

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#1552742

On 6 January 2017 at 12:03, Will Deacon <will.deacon@arm.com> wrote:
> On Thu, Jan 05, 2017 at 08:49:44PM +0100, Robert Richter wrote:
>> On 05.01.17 13:22:00, Robert Richter wrote:
>> > On 05.01.17 12:08:20, Will Deacon wrote:
>> > > I really can't see how the fix causes a crash, and I couldn't reproduce
>> > > it on any of my boards, nor could any of the Linaro folk afaik. Are you
>> > > definitely running mainline with just these two patches from Ard?
>> >
>> > Yes, just both patches applied. Various other solutions were working.
>>
>> I have retested the same kernel (v4.9 based) as before and now it
>> boots fine including rtc-efi device registration (it was crashing
>> there):
>>
>>  rtc-efi rtc-efi: rtc core: registered rtc-efi as rtc0
>>
>> There could be a difference in firmware and mem setup, though I also
>> downgraded the firmware to test it, but can't reproduce it anymore. I
>> could reliable trigger the crash the first time.
>>
>> FTR the oops.
>
> Hmm, I just can't help but think you were accidentally running with
> additional patches when you saw this oops previously. For example,
> your log looks very similar to this one:
>
>   http://lists.infradead.org/pipermail/linux-arm-kernel/2016-December/473666.html
>
> but then again, these crashes probably often look alike.
>

These are quite different, in fact. In James's case, the UEFI memory
map was missing some entries, so not all memory regions that the
firmware expected to be there were actually mapped, hence the all-zero
*pte. In Robert's case, it looks like the UEFI runtime services page
tables are corrupted, i.e., *pte has RES0 bits set.

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