From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9F122CD4F3C for ; Mon, 18 May 2026 09:08:36 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DCE036B0005; Mon, 18 May 2026 05:08:35 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D761F6B0088; Mon, 18 May 2026 05:08:35 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C8C936B008C; Mon, 18 May 2026 05:08:35 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id B40066B0005 for ; Mon, 18 May 2026 05:08:35 -0400 (EDT) Received: from smtpin02.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 7AB7A40719 for ; Mon, 18 May 2026 09:08:35 +0000 (UTC) X-FDA: 84779964990.02.FCA2C75 Received: from out-180.mta1.migadu.com (out-180.mta1.migadu.com [95.215.58.180]) by imf22.hostedemail.com (Postfix) with ESMTP id 82F6CC0010 for ; Mon, 18 May 2026 09:08:33 +0000 (UTC) Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="mn/ebOCL"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf22.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.180 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1779095313; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=ZoVBgObVlHLK3fouujqDYkCHlgaAhCdl77DK23kYJ50=; b=3jG77OyPL1Gx1d5FOowCPzK1N9LEYlSvMq4mEQQiwQdYX+wFXlRb4wfvOtve+aWF0+wtId hl8Vvh1sg6NVgTEx7IhbhhkDWEA5mbzlL9Tj93D+/e7an582G6qKzFscdzCjt3O1qcdLe8 NfrKcmQU/PKhB8FjUwLusbvD7fWaYcY= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1779095313; a=rsa-sha256; cv=none; b=T9BiOyf7IfbxgFT79VW7XJ2E1sAWW0gI0k9aqvgvttHqqKrDEcJU4ECduCe3EtqQpLBOWH JUjBmlAfPilXjVk43HqJF8PhIM2D5BemhIOW/hCGR0NuODIRxZJdUxRo4o3ev972dNiPF4 XS4qHcRAo3emf2MugWBgvDP2A8gRfdA= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="mn/ebOCL"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf22.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.180 as permitted sender) smtp.mailfrom=lance.yang@linux.dev X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779095311; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ZoVBgObVlHLK3fouujqDYkCHlgaAhCdl77DK23kYJ50=; b=mn/ebOCL9V/jPWdfLunYjJIvlB1wFcJxWyM07xrG1tHb33Hi6XaPLW1xtf2RemOdOWiOfM vcEALMSjJ086jo7epDbzh8QVdkJ4eTHs+Uof/0Hb2UYHxmu8uL5cgnyJ3L085/TaeYP7pP Usf2sFrv2Mt7agvai2wgWozRGLKTjHk= From: Lance Yang To: david@kernel.org Cc: lance.yang@linux.dev, npache@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, yuzhao@google.com, usamaarif642@gmail.com, 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 Subject: Re: [RFC] mm: restrict zero-page remapping to underused THP splits Date: Mon, 18 May 2026 17:08:20 +0800 Message-Id: <20260518090820.8101-1-lance.yang@linux.dev> In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 82F6CC0010 X-Stat-Signature: saz7w5pwt87byqji55d74brhufnswkyp X-Rspam-User: X-HE-Tag: 1779095313-945388 X-HE-Meta: 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 W3ijJB2+ XQUbLlSia7AfEULWRnvvY4mx3kyO1m74FikRepJ4kQzBtKWjT3O0gZP9oEka0lIFMglpQR5d2wF2og2llyf3kzOPlSxxJcew6Fc0PWU6p9iTQ294ZicuYcTWQgVlSfMNkWSmb6x1eqJ/9ptXG0m6Fh/NFvOyryywPUmHn4NFVXYTMdn/R1Rh0Z/ci34SLhStwPbFOSvZIvWr034XnhUSoWy4h8Kj3u3vEigmWFJpmTWP+/NV5T27ylA+ywJ0bVYrSJc4y Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 18, 2026 at 10:24:26AM +0200, David Hildenbrand (Arm) wrote: >On 5/14/26 10:11, Lance Yang wrote: >> >> On Tue, May 12, 2026 at 09:02:38PM +0200, David Hildenbrand (Arm) wrote: >>> On 5/12/26 20:36, Nico Pache wrote: >>>> On Tue, May 12, 2026 at 1:05 AM David Hildenbrand (Arm) >>>> wrote: >>>> >>>> I meant before b1f202060afe ("mm: remap unused subpages to shared >>>> zeropage when splitting isolated thp"). Sorry, I should have been more >>>> specific. >>>> >>>> As in before the underutilized shrinker (and commit b1f202060afe), we >>>> had the exact problem you describe above and no way to handle it. >>>> Correct? >>> >>> Right. That's why the underused shrinker was added, to directly free pages that >>> have been over-allocated with a THP, but are actually never "used" (remain zero). >>> >>>> >>>> I guess at reclaim THPs would be split and we would swap out a zero filled page? >>> We don't necessarily split during swapout, we try to swap out the THP if >>> supported. But yes, that can happen. >>> >>> I'm more thinking about other reasons to split a THP (e.g., migration, partial >>> MADV_PAGEOUT/MADV_COLD/MADV_FREE), where the behavior would be changed. >> >> Yeah. Today any successful anon split gets TTU_USE_SHARED_ZEROPAGE. Would >> it be better to make KSM opt out, so other split paths keep the behavior >> they have today? >> >> That way we do not have to worry about changing anything else at the >> same time :D > >This is (1) here: > >https://lore.kernel.org/all/04ea0e68-de56-49c4-8c9f-1734139d5e7f@kernel.org/ > >There will still be interaction between both mechanisms. So I'd prefer that in >VM_MERGEABLE sections with KSM enabled, only KSM would take care of deduplicating. Cool. I'd go with that approach as well. It should avoid unexpected interaction between the two :) That should keep VM_MERGEABLE VMAs under KSM's control, while leaving the other split paths unchanged for now. Cheers, Lance