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

[Hanjun Guo <hanjun.guo@linaro.org>]

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.

Thanks
Hanjun

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