linux-kernel - Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire large folio

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Message-ID: <252bac83-bf5e-4c40-a696-ac093da41fa3@arm.com>
Date: Fri, 8 Mar 2024 13:45:34 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>, Barry Song <21cnbao@...il.com>,
 akpm@...ux-foundation.org, linux-mm@...ck.org
Cc: minchan@...nel.org, fengwei.yin@...el.com, linux-kernel@...r.kernel.org,
 mhocko@...e.com, peterx@...hat.com, shy828301@...il.com,
 songmuchun@...edance.com, wangkefeng.wang@...wei.com, xiehuan09@...il.com,
 zokeefe@...gle.com, chrisl@...nel.org, yuzhao@...gle.com,
 Barry Song <v-songbaohua@...o.com>, Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire
 large folio

On 08/03/2024 13:24, David Hildenbrand wrote:
>>>> This patch migrates the last subpage to a small folio and immediately
>>>> returns the large folio to the system. It benefits both memory availability
>>>> and anti-fragmentation.
>>>
>>> It might be controversial optimization, and as Ryan said, there, are likely
>>> other cases where we'd want to migrate off-of a thp if possible earlier.
>>
>> Personally, I think there might also be cases where you want to copy/reuse the
>> entire large folio. If you're application is using 16K THPs perhaps it's a
>> bigger win to just treat it like a base page? I expect the cost/benefit will
>> change as the THP size increases?
> 
> Yes, I think for small folios (i.e., 16KiB) it will be rather easy to make a
> decision. The larger the folio, the larger the page fault latency due to
> scanning, copying, modifying, which can easily turn undesirable.
> 
> At least when it comes to page reuse, I have some simple backup plans for small
> folios if I won't be able to make progress with my other approach. 

Do you mean "small large folios" here? i.e. order >= 1? If so, great!


For larger
> folios, it won't really work/be desirable, though.
>