linux-kernel - Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any order in split page

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Date:   Tue, 17 Nov 2020 16:12:03 -0500
From:   Zi Yan <ziy@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Roman Gushchin <guro@...com>, <linux-mm@...ck.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        Yang Shi <shy828301@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        David Nellans <dnellans@...dia.com>
Subject: Re: [RFC PATCH 3/6] mm: page_owner: add support for splitting to any
 order in split page_owner.

On 17 Nov 2020, at 16:05, Matthew Wilcox wrote:

> On Fri, Nov 13, 2020 at 05:38:01PM -0800, Roman Gushchin wrote:
>> On Fri, Nov 13, 2020 at 08:08:58PM -0500, Zi Yan wrote:
>>> Matthew recently converted split_page_owner to take nr instead of order.[1]
>>> But I am not
>>> sure why, since it seems to me that two call sites (__split_huge_page in
>>> mm/huge_memory.c and split_page in mm/page_alloc.c) can pass the order
>>> information.
>>
>> Yeah, I'm not sure why too. Maybe Matthew has some input here?
>> You can also pass new_nr, but IMO orders look so much better here.
>
> If only I'd written that information in the changelog ... oh wait, I did!
>
>     mm/page_owner: change split_page_owner to take a count
>
>     The implementation of split_page_owner() prefers a count rather than the
>     old order of the page.  When we support a variable size THP, we won't
>     have the order at this point, but we will have the number of pages.
>     So change the interface to what the caller and callee would prefer.

There are two callers, split_page in mm/page_alloc.c and __split_huge_page in
mm/huge_memory.c. The former has the page order. The latter has the page order
information before __split_huge_page_tail is called, so we can do
old_order = thp_order(head) instead of nr = thp_nr_page(head) and use old_order.
What am I missing there?

Thanks.

—
Best Regards,
Yan Zi

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