Re: [PATCH v2 5/7] mm: thp: split huge page to any lower order pages.

[Zi Yan <ziy@nvidia.com>]

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On 8 Aug 2023, at 5:01, David Hildenbrand wrote:

> On 29.03.23 03:17, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> To split a THP to any lower order pages, we need to reform THPs on
>> subpages at given order and add page refcount based on the new page
>> order. Also we need to reinitialize page_deferred_list after removing
>> the page from the split_queue, otherwise a subsequent split will see
>> list corruption when checking the page_deferred_list again.
>>
>> It has many uses, like minimizing the number of pages after
>> truncating a huge pagecache page. For anonymous THPs, we can only split
>> them to order-0 like before until we add support for any size anonymous
>> THPs.
>>
>
> Because I'm currently looking into something that would also not be compatible with order-1 for now:
>
> You should make it clear that order-1 is not supported, like:
>
> "mm: thp: split huge page to any lower order pages (except order 1)"
>
> And clarify in the subject why that is the case.

Sure.

I will add below to the commit message and the comment of
split_huge_page_to_list_to_order():

Order-1 folio is not supported because _deferred_list, which is used by
partially mapped folios, is stored in subpage 2 and an order-1 folio only
has subpage 0 and 1.


--
Best Regards,
Yan, Zi

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