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Message-ID: <252bac83-bf5e-4c40-a696-ac093da41fa3@arm.com>
Date: Fri, 8 Mar 2024 13:45:34 +0000
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Subject: Re: [PATCH v2] mm: prohibit the last subpage from reusing the entire
 large folio
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To: David Hildenbrand <david@redhat.com>, Barry Song <21cnbao@gmail.com>,
 akpm@linux-foundation.org, linux-mm@kvack.org
Cc: minchan@kernel.org, fengwei.yin@intel.com, linux-kernel@vger.kernel.org,
 mhocko@suse.com, peterx@redhat.com, shy828301@gmail.com,
 songmuchun@bytedance.com, wangkefeng.wang@huawei.com, xiehuan09@gmail.com,
 zokeefe@google.com, chrisl@kernel.org, yuzhao@google.com,
 Barry Song <v-songbaohua@oppo.com>, Lance Yang <ioworker0@gmail.com>
References: <20240308092721.144735-1-21cnbao@gmail.com>
 <519fd6a7-072e-43a2-a9a8-2467ee783524@redhat.com>
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 <8e994734-1d3a-4c51-9c0b-4e2ce945f198@redhat.com>
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <8e994734-1d3a-4c51-9c0b-4e2ce945f198@redhat.com>
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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.
>