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b=XtCjeKbBbwdrOYSNwSZpOoahoJnrEqa82MCYqCKqmKloTIf0sZp4TYx7Gx2rh1ZSt ma7eDffRa0uxhKbkNhmG3howbtf2YPKu7IcfbYYVfeSqv8wEtqzLggeLV4OJO/a6V8 R0aUALt1x0gLNMc/w3uRLkHuNlkHqHeascMzgatTM/8o1PXtIi2cnhyCCl4G5t9X34 PBvRsCPyRUo3/ydRByO3SK28M4Cll3JrqkQ3WI6H8Hg1+GYRtyDGfqYIVKMXy6Kwcs goLwr3uzs2HJ2sfMHzpss3xEK/47dBJ4z1OGTYmNFVN3oEFagD+Z6TQacB/nVPrpGZ qNcFMAh+gqVmw== Message-ID: <46914256-508a-4737-86b9-a43be7e628a7@kernel.org> Date: Mon, 21 Sep 2026 12:23:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/3] mm: split underused anonymous mTHP folios To: Joanne Koong , akpm@linux-foundation.org, ljs@kernel.org Cc: usama.arif@linux.dev, hannes@cmpxchg.org, baohua@kernel.org, alex@ghiti.fr, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, lance.yang@linux.dev, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, willy@infradead.org, linux-mm@kvack.org References: <20260916224437.1164512-1-joannelkoong@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: yhf7jt5yga9dhbqobirj5ibpwnknofmc X-Rspam-User: X-Rspamd-Queue-Id: A44421C0004 X-Rspamd-Server: rspam03 X-HE-Tag: 1789986200-384314 X-HE-Meta: U2FsdGVkX18v1mun3n6D3pGUqguVEoq2H/6HAVVcwzDXGdZqcs65nYyOcVQ8sFX0wWY0CH3Y+8qdw3/EbOheWdZINF8urxF7yMArT26q4rSFPPbIIqNuiAOt1Dxyna4gd6bUtWO0NZrA9lLb3BL6QGBvkMfd51U5Z4O5zTyWiYvtIKYKX4SkEnWJ6m1+zmXvBgb4ni9ChjKIS/k5CT/FEpDl3qpQ9qUO6LrUoLALoqDmrs1eEv23J9HRIXePNe5vX26RC6Jbq7mJNoQNp07+XML6cURf4FxpDp6+1QU38mKuwcxbjHoBsrfMa0Lt0oWlogwl/ZLR9Z7cpGfwdeym0/sn3bwPeXWiKotfizINXqfJjt43ms4b6NpaBJu7i3PDhHvvcwGvZRWHsC00w8h0j1Es6TjrWIsZb40fjRPazwopHDrYECgL/RqgWgVzt+fGp2GaUz4EQVCCwMhewJ+gpVhVi5T5iHXn5D1Hpq4yp2OMFdUySuz8O0qimCPoRmVJLbRrlY5XsLFOQ6MbBw91HsIgakmIV3rBhdniuPms07jMzeiTnRs3LAQIQMy2sGmwMTTx2sitZK8IOS8S88aK6vvM6se5tK1Cv2790QfMbLwLJMXKSp7Dp97uJolK64lLo2eO7k39oduZ7zftMf45yVH6lX6F6PbviKQ/e8ms3z8ZnZ1aKndivamm84/6wMAuPu+B6RaKkYr4bMoTDjQqhlDeGZf7dc4u4YgLUxccv6eD81wzIYT30c9sfOZjLMRFd2hcwxPJsu1bz6ChiiZVAfNGZS42VceoeUqkTNXXalozNI65afln11z0UjYqBtOYECajPbvqR3YBL6rK3fOhHIMAC35HCePFLdNpqMumh3920hxnndC8rYv3nX1T2j/aPJBX8GiNl2FJihxXK1EyTPr3qgEIAZC7fSPCVYDVAVxWhGmg1ze/du7+c+/zBzVYdCBX1s0L/n7pSfWOba2 f68J5ycx E2JGARPg5rbXAhKIbIxNE9gtKhgc7oMpzLlUd6b5v88KqQaJPR7V+PEbJ8UJwe00jV65C0DzC3rzgwoURhaH3SUo1wge43imkSOfnczfM8/LXRe2doIIxrqpUHcYKAFNmZTCFoGGiOkolLl7ESCIZ1WZ7SQiuYJ/Pmp+Ay+nTs4Lm1bCyEWNiL047Emzb7Sa0kwbQZRC6aCP4KZR4J6dQK6S0ib2a6YGWPI3vf7keBn/OYZ4L+8OcOG+7sAyLTu9xWD0b Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/17/26 00:44, Joanne Koong wrote: > PMD-sized THPs that are mostly zero-filled are reclaimed under memory > pressure by the deferred split shrinker, but this is not done for mTHP > folios. At Meta we would like to deploy 2M THP=always on arm64 with 64k > base pages, as 2M provides the contpte benefits while the PMD size there > (512M) is too big to use. However, 2M THP=always causes memory regressions > unless the unused portions of those folios can be broken down and > reclaimed. > > v1 did two things. It scaled khugepaged_max_ptes_none down per folio order and > it queued every anonymous mTHP folio. David objected to the scaling since > collapse had already rejected proportional scaling as either letting the memory > footprint creep or being confusing to reason about. David and Barry separately > objected to the queuing, which adds list_lru lock contention on every anonymous > fault, and hurts lower-order use cases such as Android's order-2-only > configuration. Johannes suggested keeping khugepaged_max_ptes_none absolute and > simply not queuing folios that could never exceed it, which addresses both. > > Read as an absolute count, khugepaged_max_ptes_none also implies the > smallest folio that takes part in underused splitting. A 2M folio on 64k > base pages is 32 pages, so the knob has to be set below 32 for those to > be queued at all, and any folio with no more pages than the value stays off > the queue entirely. > > Patch 1 makes thp_underused() folio-size aware. Today it only ever sees > PMD-sized folios, so it assumes HPAGE_PMD_NR pages throughout. Patch 3 > breaks that assumption, so patch 1 generalizes it first. No functional > changes are introduced. > > Patch 2 keeps folios that can never be found underused off the deferred > split queue, as suggested by Johannes. This changes/optimizes existing > behavior. With the default khugepaged/max_ptes_none, PMD folios are queued > today and then dropped again by the first scan without ever having been > splittable. After this patch they are not queued at all. I think we discussed queuing folios once the value changes and you would actually be able to split some. Was there a good reason why that idea was dropped? -- Cheers, David