Re: [PATCH v3] arm64: mm: Fix NOMAP page initialization

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 6 January 2017 at 01:07, Hanjun Guo <hanjun.guo@linaro.org> wrote:
> On 2017/1/5 10:03, Hanjun Guo wrote:
>>
>> On 2017/1/4 21:56, Ard Biesheuvel wrote:
>>>
>>> On 16 December 2016 at 16:54, Robert Richter <rrichter@cavium.com> wrote:
>>>>
>>>> On ThunderX systems with certain memory configurations we see the
>>>> following BUG_ON():
>>>>
>>>>  kernel BUG at mm/page_alloc.c:1848!
>>>>
>>>> This happens for some configs with 64k page size enabled. The BUG_ON()
>>>> checks if start and end page of a memmap range belongs to the same
>>>> zone.
>>>>
>>>> The BUG_ON() check fails if a memory zone contains NOMAP regions. In
>>>> this case the node information of those pages is not initialized. This
>>>> causes an inconsistency of the page links with wrong zone and node
>>>> information for that pages. NOMAP pages from node 1 still point to the
>>>> mem zone from node 0 and have the wrong nid assigned.
>>>>
>>>> The reason for the mis-configuration is a change in pfn_valid() which
>>>> reports pages marked NOMAP as invalid:
>>>>
>>>>  68709f45385a arm64: only consider memblocks with NOMAP cleared for
>>>> linear mapping
>>>>
>>>> This causes pages marked as nomap being no longer reassigned to the
>>>> new zone in memmap_init_zone() by calling __init_single_pfn().
>>>>
>>>> Fixing this by implementing an arm64 specific early_pfn_valid(). This
>>>> causes all pages of sections with memory including NOMAP ranges to be
>>>> initialized by __init_single_page() and ensures consistency of page
>>>> links to zone, node and section.
>>>>
>>>
>>> I like this solution a lot better than the first one, but I am still
>>> somewhat uneasy about having the kernel reason about attributes of
>>> pages it should not touch in the first place. But the fact that
>>> early_pfn_valid() is only used a single time in the whole kernel does
>>> give some confidence that we are not simply moving the problem
>>> elsewhere.
>>>
>>> Given that you are touching arch/arm/ as well as arch/arm64, could you
>>> explain why only arm64 needs this treatment? Is it simply because we
>>> don't have NUMA support there?
>>>
>>> Considering that Hisilicon D05 suffered from the same issue, I would
>>> like to get some coverage there as well. Hanjun, is this something you
>>> can arrange? Thanks
>>
>>
>> Sure, we will test this patch with LTP MM stress test (which triggers
>> the bug on D05), and give the feedback.
>
>
> a update here, tested on 4.9,
>
>  - Applied Ard's two patches only
>  - Applied Robert's patch only
>
> Both of them can work fine on D05 with NUMA enabled, which means
> boot ok and LTP MM stress test is passed.
>

Thanks a lot Hanjun.

Any comments on the performance impact (including boot time) ?

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