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 B4F8BCFD379 for ; Tue, 25 Nov 2025 08:52:33 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 04B016B0024; Tue, 25 Nov 2025 03:52:33 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id F3DC56B0026; Tue, 25 Nov 2025 03:52:32 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E2C466B0027; Tue, 25 Nov 2025 03:52:32 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id D0CFD6B0024 for ; Tue, 25 Nov 2025 03:52:32 -0500 (EST) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 80BE21A0766 for ; Tue, 25 Nov 2025 08:52:32 +0000 (UTC) X-FDA: 84148513344.07.551580C Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf19.hostedemail.com (Postfix) with ESMTP id BF7FB1A000D for ; Tue, 25 Nov 2025 08:52:30 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=jlWqvyTf; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1764060750; a=rsa-sha256; cv=none; b=GenFLRwd5BGMp+CBgNM4FLJtJrQKn58Gp7WGZnplL/F2lU8E0w+ZtQk76+1ylDKVLAmOPt KP9Q07zc6M3qChIhjFRfhZeY24FX4FReGQthRNm4LbjduIsP/y2SDs0xkm6KDXhMzsIZDH 5gE7vf1XSosNvYJX401sH6YWUJseIjY= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=jlWqvyTf; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1764060750; 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=XxgCjm8KJfnOG75IlYZIKmCNP4rFm4UYg08/PhxOZO0=; b=VPsokUiJJBL2TUZxxuRQ5XO43cDgGv+oj5xiGq1FRNJEijAUMjBY0WUJ9e2ZAg1j4Q6+J2 DBC1B6LtXgswg91WKh9RFmQkcomNZcephIMckvRkimKPBOxyb/W70lSiATY7ZU9LtULyx8 C04yFXCjm9l76FWssha4JFGHhLU3AGo= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 029886014B; Tue, 25 Nov 2025 08:52:30 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A900C116D0; Tue, 25 Nov 2025 08:52:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764060749; bh=WACZF/HwQACWdEWXrmIS+prCAYLYXVe/fT2oiuOjUjc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=jlWqvyTfEjzLn7bT1JbFiKyTwqaxXFDroS868o8NNusJwRiTK+Sps76NoS0cLLPvc f0Zez5qO/g7UB3ZtE2LfiYzfMJwZrRFFfIvfQV3VBfQOBUdfZpNVioCAybcC4IRzIY LNhKW3xwaKQ3dlvcyzflVtWWnLfpJFAcKi+1MyMiWGp94SxVAwDhapEMqMZteLOLF6 j/6pttGboL2g6ZZLWd6UAiOZOwkHAesMCVAVmmZegLPh3bVgNpifhyjup4oWXLRZNq Jm4/S4AOfkg9xz2wfe4M4kkA1VrtxrD011ACQXUYgsRwS1aDd3UyMpqsyRlAASJaz9 r+C9omdYQxaew== Message-ID: <2f708eae-7d4b-43b0-83f0-7c2d98b294e6@kernel.org> Date: Tue, 25 Nov 2025 09:52:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/4] mm/huge_memory: replace can_split_folio() with direct refcount calculation To: Zi Yan Cc: Lorenzo Stoakes , Andrew Morton , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Miaohe Lin , Naoya Horiguchi , Wei Yang , Balbir Singh , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251122025529.1562592-1-ziy@nvidia.com> <20251122025529.1562592-3-ziy@nvidia.com> <33A929D1-7438-43C1-AA4A-398183976F8F@nvidia.com> <34bafd06-250a-4019-8b34-5ddeedea1cb3@kernel.org> <66C159D8-D267-4B3B-9384-1CE94533990E@nvidia.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <66C159D8-D267-4B3B-9384-1CE94533990E@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: BF7FB1A000D X-Stat-Signature: t5b8agddp1ufderi9ku5gsiw1m1jpojk X-HE-Tag: 1764060750-443481 X-HE-Meta: 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 gkz/IgLm hK65Js+LciDrxcxGVebZrJjXwrv+Ujyc0C4K2CGxLTVBd15g0O6xBd2r2DSzI09zZICknA03r3CUqPM3X+lPONjRB4jbIYguzfS8SCTpArZiiOWZTMzXGzLbVyri5wbh9yTn6rn/o7PoSa7woekjyAny46xSKyQ2Qs8ddA7CoiV+hYne/kOzCrNaUZ5/3Aij/yBKgYS89lAf7JBaekN4Ct975lrgkquq/UMHPHSDBQFtonnPcAckfjgL3vFBgDWZ2TGU2Uj/wgUI3rD3aOPU5N+yZoDcKbbPuo5JDykUvNpCns1drWtpzoTrQZUyN/dgsJpGC4ppqDdik+DMaVu5aE2621F6VBG8Ld8tE8NdY01caWwgJfx4n7nHgw2PwaWRpr4UBgXiAueIK4k9u8fLpLKXqZg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> >>> >>> Like: >>> >>> if (folio_test_anon(folio)) { >>> /* One reference per page from the swapcache. */ >>> ref_count += folio_test_swapcache(folio) << order; >>> } else { >>> /* One reference per page from shmem in the swapcache. */ >>> ref_count += folio_test_swapcache(folio) << order; >>> /* One reference per page from the pagecache. */ >>> ref_count += !!folio->mapping << order; >>> /* One reference from PG_private. */ >>> ref_count += folio_test_private(folio); >>> } >>> >>> or simplified into >>> >>> if (!folio_test_anon(folio)) { >>> /* One reference per page from the pagecache. */ >>> ref_count += !!folio->mapping << order; >>> /* One reference from PG_private. */ >>> ref_count += folio_test_private(folio); >>> } >>> /* One reference per page from the swapcache (anon or shmem). */ >>> ref_count += folio_test_swapcache(folio) << order; >>> ? >> >> That is incorrect I think due to swapcache being able to give false positives (PG_owner_priv_1). > > Got it. So it should be: > > if (folio_test_anon(folio)) { > /* One reference per page from the swapcache. */ > ref_count += folio_test_swapcache(folio) << order; > } else { > /* One reference per page from shmem in the swapcache. */ > ref_count += (folio_test_swapbacked (folio) && > folio_test_swapcache(folio)) << order; > /* One reference per page from the pagecache. */ > ref_count += !!folio->mapping << order; > /* One reference from PG_private. */ > ref_count += folio_test_private(folio); > } Interestingly, I think we would then also take proper care of anon folios in the swapcache that are not anon yet. See __read_swap_cache_async(). I wonder if we can clean that up a bit, to highlight that PG_private etc do not apply. if (folio_test_anon(folio)) { /* One reference per page from the swapcache. */ ref_count += folio_test_swapcache(folio) << order; } else if (folio_test_swapbacked (folio) && folio_test_swapcache(folio)) { /* to-be-anon or shmem folio in the swapcache (!folio->mapping) */ ref_count += 1ul << order; VM_WAN_ON_ONCE(folio->mapping); } else { /* One reference per page from the pagecache. */ ref_count += !!folio->mapping << order; /* One reference from PG_private. */ ref_count += folio_test_private(folio); } Or maybe simply: if (folio_test_swapbacked (folio) && folio_test_swapcache(folio)) { /* * (to-be) anon or shmem (!folio->mapping) folio in the swapcache: * One reference per page from the swapcache. */ ref_count += 1 << order; VM_WAN_ON_ONCE(!folio_test_anon(folio) && folio->mapping); } else if (!folio_test_anon(folio)) { /* One reference per page from the pagecache. */ ref_count += !!folio->mapping << order; /* One reference from PG_private. */ ref_count += folio_test_private(folio); } > > I wonder if we should have folio_test_shmem_in_swapcache() instead. Interestingly, thinking about it, I think it would also match to-be anon folios and anon folios. folio_in_swapcache() maybe ? > > BTW, this page flag reuse is really confusing. Yes ... > I see PG_checked is > PG_owner_priv_1 too and __folio_migrate_mapping() uses folio_test_swapcache() > to decide the number of i_pages entries. Wouldn’t that cause any issue? Maybe at that point all false positives were ruled out? It is horrible TBH. -- Cheers David