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b=XJ/vawllytAbPYs8p9gupcoh65VusCSetitLW/T3z5kxZQimzbMtjSspkFQXH+rhO ReEgxG47Mz3+OOtvGqsxJVMtX5BllLLfXWRGCpmRvjxpkn2g7OfbWWMUNuXPOPqRkP F6EwgkNyVZ3yDfcUWCxtktji6mk0OmK5QE4nX0zxOaFe6gWmlfdHNn9eFhFHauH9wW Q9KMn3wKvTfK6KB85kqheOC4eUJIrewDVvPimIwjjrCagcgplwfCWvB1e7rzBsiEJB s8HdyAOf4C4WaKevKFVILiOZs/dresHyf0hKH9EDWJQTnytWh18ZlagYBwDejxTPUi PR6x6LMGTzKgA== Message-ID: <49a274cf-56f7-4d16-aa49-b2e5b8eae196@kernel.org> Date: Tue, 22 Sep 2026 20:59:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/3] mm: split underused anonymous mTHP folios To: Joanne Koong Cc: akpm@linux-foundation.org, ljs@kernel.org, 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> <46914256-508a-4737-86b9-a43be7e628a7@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: bocpbkmuopg4ykretr649fiwisaz1nn8 X-Rspamd-Queue-Id: 788694000C X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790103586-30608 X-HE-Meta: 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 HD+rPUmq D8xf4m1khfaYrb4EedqCHvqOTyxIRMwS2WA93jEjZRu/CN9E5o8F9cthO1DURw55tgMGJrQsZd2TB0fmNqYe0rTo5fd3l/2kd5NespxIHnAI2dvOtW8DpoezERytk7lhToKD+/C1YC3JUAoCQjkjMVpCj/f9wiITaQPFw4F3kJms3CLLtWMhQauaC6+oTEU2AZGUOIKFrPAlG6Fu03Zvk/WCBCcAxRVLIDz6WiqdAg9mNQ1kRETgg9gk8TzbdbniYfn35uyGBKCI2GK0sgP8X7nBzLX+UP0njwU/L5ZmnxqeJcE9pwCWZRT8Jrw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/21/26 22:33, Joanne Koong wrote: > On Mon, Sep 21, 2026 at 3:23 AM David Hildenbrand (Arm) > wrote: >> >> 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? >> > > Hi David, > > I have a paragraph about this in the commit message of patch 2: > Ah, okay, I only skimmed the cover letter so far. -- Cheers, David