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

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Message-ID: <3A55517E-7F24-45ED-A04B-061948E7EC11@nvidia.com>
Date:   Tue, 17 Nov 2020 16:25:46 -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 16:22, Matthew Wilcox wrote:

> On Tue, Nov 17, 2020 at 04:12:03PM -0500, Zi Yan wrote:
>> 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?
>
> Sure, we could also do that.  But what I wrote was true at the time I
> wrote it.

Got it. Thanks. Will change it to use old_order to make split_page_owner parameters
look more consistent.

—
Best Regards,
Yan Zi

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