linux-kernel - Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

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Message-ID: <c9c98cbf-711c-4755-ae99-fb13aeb51381@redhat.com>
Date: Fri, 5 Jul 2024 12:55:58 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 dj456119@...il.com, ryan.roberts@....com, shy828301@...il.com,
 ziy@...dia.com, libang.li@...group.com, baolin.wang@...ux.alibaba.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Mingzhe Yang <mingzhe.yang@...com>
Subject: Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

On 05.07.24 12:48, Lance Yang wrote:
> Hi David and Barry,
> 
> Thanks a lot for paying attention!
> 
> On Fri, Jul 5, 2024 at 6:14 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 05.07.24 12:12, Barry Song wrote:
>>> On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>                 i_mmap_unlock_read(mapping);
>>>>>     out:
>>>>>         xas_destroy(&xas);
>>>>> -     if (is_thp)
>>>>> +     if (order >= HPAGE_PMD_ORDER)
>>>>
>>>> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
>>>> future.
>>>
>>> I feel this might need to be separate since all other places are using
>>> folio_test_pmd_mappable() ?
>>
>> Likely, but as you are moving away from this ... this counter here does
>> and will always only care about HPAGE_PMD_ORDER.
> 
> I appreciate the different opinions on whether we should use
> ">= HPAGE_PMD_ORDER" or "==" for this check.
> 
> In this context, let's leave it as is and stay consistent with
> folio_test_pmd_mappable() by using ">= HPAGE_PMD_ORDER",
> what do you think?

I don't think it's a good idea to add more wrong code that is even 
harder to grep (folio_test_pmd_mappable would give you candidates that 
might need attention). But I don't care too much. Maybe someone here can 
volunteer to clean up these instances to make sure we check PMD-size and 
not PMD-mappable for these counters that are for PMD-sized folios only, 
even in the future with larger folios?

-- 
Cheers,

David / dhildenb