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b=VPQQZ+i03zE5V0qYX1spklVsimLKMkU5UsLI7UI4NbPJYbmI0UepTG07Rut9hAWQp 01xmLtnJBrCX2k636dY1vqD/8egVxf5W5cKEx34gyy1/e6tEejEMmS49B2TZI0Z0xz DSaWOHk7eGW+DBqG07WukoaxVLSBCOwsEOA0KQIbniMoUG7rcNiRP9sARy7d/EsBJs 2CPfygwPsAPSYmhoKajPOeNk/FCEq/GIZzlIdwAgQnDQ9+frDLV34PpEJBa6d5A7hV HgEqb3VRFQJUlDBDxzzAxF/Xe3LQBe+HRae1i4PReyRbG8OHmqDuX7+hvp8MwnCgny HLPfeUd5SsjVA== Message-ID: Date: Mon, 11 May 2026 15:44:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mm: restrict zero-page remapping to underused THP splits From: "David Hildenbrand (Arm)" To: Usama Arif Cc: Nico Pache , linux-mm@kvack.org, linux-kernel@vger.kernel.org, yuzhao@google.com, usamaarif642@gmail.com, lance.yang@linux.dev, baohua@kernel.org, dev.jain@arm.com, ryan.roberts@arm.com, liam@infradead.org, baolin.wang@linux.alibaba.com, ziy@nvidia.com, ljs@kernel.org, akpm@linux-foundation.org References: <20260510114001.600681-1-usama.arif@linux.dev> <8838114e-5b6a-4f3d-932c-9e97e51216ae@kernel.org> <608bef55-44d1-47f1-a201-4a6bd7be137d@linux.dev> <574fc329-bf2d-4686-9f15-b1709432326e@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 946F720018 X-Stat-Signature: h19iskbgbmfii4s1z5yiwgd76t6s7kfh X-Rspam-User: X-HE-Tag: 1778507087-230030 X-HE-Meta: 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 U0VAlXz8 RJuTlKEL4gllOikBsi4ytv8fQncP++bxFauRoyX1gkCKOo3xC2u6h3vT6KFcivdQfP4A3f5PhYVqFN5s8tZzlS2KcYB445BU41SUkOXpoC5QJYZyCZiXwgw5LUdvIb0Ov3nEEuAp2jsGKWWsAYZea5PQa+uMNBvh3T/DhHH5SNi8sSfBS4KMZa7ck5RArcQCR6FVW8KqUcgrfLPHyR6kVqYE+HswLxNiqGpOfJ8vJyAHXhasSyvZM/UZRwY+VQl4s55DoHZKPZeWgdIqy4Z1EZjoSBp3ZTcNHynwK Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/26 15:42, David Hildenbrand (Arm) wrote: > On 5/11/26 15:10, Usama Arif wrote: >> >> >> On 11/05/2026 07:36, David Hildenbrand (Arm) wrote: >>> >>> >>> >>> Hi! >>> >>> >>> Yes, but that's what the users ask for: if there is a chance to deduplicate >>> memory, it shall be deduplicated asap. >>> >>> >>> Right, the probability is low, and it would change existing semantics, breaking >>> existing users. >>> >>> In addition, we would have to add large folio support for KSM, which I rather >>> would avoid. >>> >>> >>> Right, but that's what you get with KSM: bad performance if there is a chance to >>> deduplicate :) >>> >>> (and bad performance from scanning overhead) >>> >>> >>> It's not just the zero page, but really any page content. The zero page is >>> currently only "special" after we added conditional support to deduplicate to >>> the shared zeropage in KSM. The shrinker doesn't help for any other page content >>> besides zero-filled. >>> >>> Further, the shrinker is something system-wide, whereby KSM is usually only >>> enabled for selected VMAs (with some exceptions nowadays). >>> >>> Also note that KSM deduplicates independent of the folio size: not just THPs, >>> but really any (large) folio. Yes, it splits large folios, but that's really >>> just to keep the T in THP. >>> >>> >>> Right, and it makes KSM more THP aware. Which is something I would avoid right now. >>> >>> >>> That would change the semantics where, for example, where we expect that memory >>> was deduplicated after a KSM run. >>> >>> VMs (where KSM is usually employed) are expected to be mostly backed by THPs: >>> except where we can deduplicate memory. Skipping THPs would essentially break >>> the main use case for KSM :) >>> >>> Does that make sense? >>> >> >> Yes, all of above makes sense. But I feel like this means someone should not >> set THP policy to always and enable KSM together. > > IIRC, QEMU will, as default, set MADV_HUGEPAGE and MADV_MERGEABLE :) > > (KSM itself later has to be enabled manually on a system level) Digging around, RHEL documents that one might want to consider disabling THPs for performance: "As KSM can reduce the occurence of transparent huge pages, you may want to disable it before enabling THP." [1] But that doesn't mean that some people are using that combination. In the end "some THPs" is better than "no THPs" :) [1] https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/virtualization_tuning_and_optimization_guide/sect-virtualization_tuning_optimization_guide-memory-huge_pages -- Cheers, David