linux-kernel - Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

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Message-ID: <f4c3972e-f341-4d15-baa2-bedda8d7a8ad@arm.com>
Date:   Tue, 28 Nov 2023 15:34:50 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

On 28/11/2023 14:09, David Hildenbrand wrote:
> On 28.11.23 13:15, Ryan Roberts wrote:
>> On 28/11/2023 08:48, David Hildenbrand wrote:
>>>
>>>>>
>>>>> Agreed. We are bikeshedding here. But if we really can't swallow "small-sized
>>>>> THP" then perhaps the most efficient way to move this forwards is to review
>>>>> the
>>>>> documentation (where "small-sized THP" appears twice in order to differentiate
>>>>> from PMD-sized THP) - its in patch 3. Perhaps it will be easier to come up
>>>>> with
>>>>> a good description in the context of those prose? Then once we have that,
>>>>> hopefully a term will fall out that I'll update the commit logs with.
>>>>>
>>>>
>>>> I will see you over in patch 3, then. I've already looked at it and am going
>>>> to suggest a long and a short name. The long name is for use in comments and
>>>> documentation, and the short name is for variable fragments:
>>>>
>>>>        Long name:  "pte-mapped THPs"
>>>>        Short names: pte_thp, or pte-thp
>>>
>>> The issue is that any THP can be pte-mapped, even a PMD-sized THP. However, the
>>> "natural" way to map a PMD-sized THP is using a PMD.
>>>
>>
>> How about we just stop trying to come up with a term for the "small-sized THP"
>> vs "PMD-sized THP" and instead invent a name that covers ALL THP:
>>
>> "multi-size THP" vs "PMD-sized THP".
>>
>> Then in the docs we can talk about how multi-size THP introduces the ability to
>> allocate memory in blocks that are bigger than a base page but smaller than
>> traditional PMD-size, in increments of a power-of-2 number of pages.
> 
> So you're thinking of something like "multi-size THP" as a feature name, and
> stating that for now we limit it to <= PMD size. mTHP would be the short name?

Sure.

> 
> For the stats, we'd document that "AnonHugePages" and friends only count
> traditional PMD-sized THP for historical reasons -- and that AnonHugePages
> should have been called AnonHugePmdMapped (which we could still add as an alias
> and document why AnonHugePages is weird).

Sounds good to me.

> 
> Regarding new stats, maybe an interface that indicates the actual sizes would be
> best. As discussed, extending the existing single-large-file statistics might
> not be possible and we'd have to come up with a new interface, that maybe
> completely lacks "AnonHugePages" and directly goes for the individual sizes.

Yes, but I think we are agreed this is future work.