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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Yang Shi <shy828301@gmail.com>, Yu Zhao <yuzhao@google.com>,
        linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        Roman Gushchin <roman.gushchin@linux.dev>,
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        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v2 5/7] mm: thp: split huge page to any lower order pages.
Date:   Tue, 08 Aug 2023 10:45:34 -0400
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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 spli=
t
>> them to order-0 like before until we add support for any size anonymou=
s
>> THPs.
>>
>
> Because I'm currently looking into something that would also not be com=
patible 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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