Re: [PATCH v2 0/5] Use nth_page() in place of direct struct page manipulation

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On 8 Sep 2023, at 10:46, Philippe Mathieu-Daudé wrote:

> Hi,
> On 6/9/23 17:03, Zi Yan wrote:
>> From: Zi Yan <ziy@xxxxxxxxxx>
>> On SPARSEMEM without VMEMMAP, struct page is not guaranteed to be
>> contiguous, since each memory section's memmap might be allocated
>> independently. hugetlb pages can go beyond a memory section size, thus
>> direct struct page manipulation on hugetlb pages/subpages might give
>> wrong struct page. Kernel provides nth_page() to do the manipulation
>> properly. Use that whenever code can see hugetlb pages.
> How can we notice "whenever code can see hugetlb pages"?
> From your series it seems you did a manual code audit, is that correct?
> (I ask because I'm wondering about code scalability and catching other
> cases).

Anything allocated from buddy allocator should be free of this problem,
because MAX_ORDER is always smaller than a memory section size. This means
majority of kernel code should be fine. What is left is core mm code that
can have a chance to touch hugetlb, like migration, memory compaction,
and of course hugetlb code. Yes, I did a manual code audit. And hopefully
I caught all cases.

An alternative is to use nth_page() everywhere, but that is a very invasive
change for an uncommon config (SPARSEMEM + !VMEMMAP).

Best Regards,
Yan, Zi

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