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]) by smtp.lore.kernel.org (Postfix) with ESMTP id 67757C25B4E for ; Tue, 24 Jan 2023 10:17:51 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C6D576B0074; Tue, 24 Jan 2023 05:17:50 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C1D766B0075; Tue, 24 Jan 2023 05:17:50 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id ABE3D6B0078; Tue, 24 Jan 2023 05:17:50 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 9B4A66B0074 for ; Tue, 24 Jan 2023 05:17:50 -0500 (EST) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 6C1C01209E1 for ; Tue, 24 Jan 2023 10:17:50 +0000 (UTC) X-FDA: 80389291500.13.8278DA9 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf01.hostedemail.com (Postfix) with ESMTP id 399BC4001E for ; Tue, 24 Jan 2023 10:17:48 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=HsszXucB; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf01.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1674555468; 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=krTIstqgNa3Jm9HB44XJKCzpTg02BS6CTQidrfRJNkk=; b=C9IU2WO7pp+2uNe4ZBET8saNKC0x2ufqpPu+ReIXRdvZ31eHI7rGsS2MTEWz6XvXRCh6On 8eIkTW4nMa2uEOCQ6/kR4eNsIC4SJ8/VQ04UvtmAQ0rput5tawLBShKCTpQZiHyNjYWJ5s 8f/UQYn3UHzPAaJ/vrToxfSWG1acNoc= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=HsszXucB; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf01.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1674555468; a=rsa-sha256; cv=none; b=v/t022m3zsOB4mmxKogDPWzoecBJQXP098qbjgZo0B8TLsgyQf7Jcb0NolCHJeHPurazt7 HxoBktIG7A2iAyqVbWQvTJ5fLQ3cKocPcKRirdbo8Ll/RfiayM1VyWdsLX3kxzr+70K4P3 fdLJ7aeJVIihY1BQTSAneuMCD1LY7dQ= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1674555467; 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=krTIstqgNa3Jm9HB44XJKCzpTg02BS6CTQidrfRJNkk=; b=HsszXucBABpDD81Tnv2GAsJREOM4SOGtbxVli/ydxPpBWstQQFXVleuBvJ7tCtDvwmVomF XcIBilsLbP/Fpy0iXeGruEsRKmeOBjmyuW0gI9RYjeOL8y4oS6vCsTrguaHfSrVsC1uy7R kDRuQeM8vQVYwoJJ8vUWNedARf3fRzc= Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-586-Iw3rtnp2MayOSWll0Trqig-1; Tue, 24 Jan 2023 05:17:46 -0500 X-MC-Unique: Iw3rtnp2MayOSWll0Trqig-1 Received: by mail-wr1-f70.google.com with SMTP id k18-20020adfb352000000b002bdd0a7a2b5so2485969wrd.0 for ; Tue, 24 Jan 2023 02:17:46 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:in-reply-to:organization:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=krTIstqgNa3Jm9HB44XJKCzpTg02BS6CTQidrfRJNkk=; b=cCLgv/kE3VMVsBTS+N3IqXWRJ4dJtZHLMqTpRQ2dBS+uRWfzur+uI5Il8ZPDgvBgpw Qy5AQgh/+vw7pLIs3h22BXvSCP9fPX4GY8gU4fTWJHqszQLD+1JuqjysqGFYKdkRaHRV +AXoz6BabVseV1MROqWtVU4nK4LWmcr8klscVof6fNTf2Q2XHTBPb3uZ/Pic413l38Z+ u1EBjoKMufoUrgltvHMG5fZTdqIiy5cudhDrlkPO03caB9EBjFpPxhDAlj94sFT+oKHd dv2JcrX0Zf+VBc0haeNVvtF0eduzU2q94+gXKBbjuj5Doz9LklXFjCkJx5juYRlYHCrW qEYw== X-Gm-Message-State: AFqh2krRFJMiK+qA57sfm/6hAujOz9UFDePJ+oiS5ERN//VuHCxZAHZl VAg+toE0o6ngjAaHc6v43qR3ZqzhlM7SP7rlocfDAnDHqtYU1cyjDLwTyGez7L5FGJ0tDUd/R0T VxVZ4Pp332gY= X-Received: by 2002:a05:6000:98d:b0:2be:129c:2920 with SMTP id by13-20020a056000098d00b002be129c2920mr27495243wrb.13.1674555464973; Tue, 24 Jan 2023 02:17:44 -0800 (PST) X-Google-Smtp-Source: AMrXdXt9sDIeFrtTARrl/I71l7DcQiEq4TDVriQ6laxq8SGedjpnBZM2KMiVZYGJTUJw+EoXZAGnrw== X-Received: by 2002:a05:6000:98d:b0:2be:129c:2920 with SMTP id by13-20020a056000098d00b002be129c2920mr27495226wrb.13.1674555464678; Tue, 24 Jan 2023 02:17:44 -0800 (PST) Received: from ?IPV6:2003:cb:c707:9d00:9303:90ce:6dcb:2bc9? (p200300cbc7079d00930390ce6dcb2bc9.dip0.t-ipconnect.de. [2003:cb:c707:9d00:9303:90ce:6dcb:2bc9]) by smtp.gmail.com with ESMTPSA id j8-20020adff008000000b00297dcfdc90fsm1541164wro.24.2023.01.24.02.17.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Jan 2023 02:17:44 -0800 (PST) Message-ID: Date: Tue, 24 Jan 2023 11:17:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in do_migrate_range To: Matthew Wilcox , Sidhartha Kumar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, osalvador@suse.de References: <20230123202347.317065-1-sidhartha.kumar@oracle.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 399BC4001E X-Stat-Signature: aawdrpdwu5oi3awq4ug1idsrn5fj3fwn X-HE-Tag: 1674555468-410364 X-HE-Meta: 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 h93c8NFR 5PfNk11G+ZrwUB/Mz3rQGob+W5IwWFzT8EgX85Mf/usflChwOuWR15K2z6qiFXrKqpKdw4H8sgJIuvvVxOuXRtM4bL2uPUL4xeAwRe7lgN7QskKNlxiAR5VWEDyvm6zAKqojTikcZBPVPRKPt56wyLTB8fm/Hbrvj3pU/NzDyfNe02bqX7ECqfZWzgAmiC40qAiozlOypwQPTYsbhNIf4WKFDD7czsIT10GBoz2jG5QzmSBla0vcy1vk7uJEII+W709gDH7NFTN3NxGwO5Q4x/f4WAPya9T2ap+JtgzFqxvE24WQO+Qob1fJuQJwzT0WDwWHHoeFmsljn539xraSSeMbIccE7/SaZfL8tqd5mQ0RjIAIoVEKC4/4VKWdtoSo48aAeAxDbtknvlR6NgTGK0t43IaHzOZXZFChZ+q32MrM78msw/1/jlgc7ziYZANrYH6cf X-Bogosity: Ham, tests=bogofilter, spamicity=0.000141, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 23.01.23 21:37, Matthew Wilcox wrote: > On Mon, Jan 23, 2023 at 12:23:46PM -0800, Sidhartha Kumar wrote: >> @@ -1637,14 +1637,13 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) >> continue; >> page = pfn_to_page(pfn); >> folio = page_folio(page); >> - head = &folio->page; >> >> - if (PageHuge(page)) { >> - pfn = page_to_pfn(head) + compound_nr(head) - 1; >> + if (folio_test_hugetlb(folio)) { >> + pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; >> isolate_hugetlb(folio, &source); >> continue; >> - } else if (PageTransHuge(page)) >> - pfn = page_to_pfn(head) + thp_nr_pages(page) - 1; >> + } else if (folio_test_transhuge(folio)) >> + pfn = folio_pfn(folio) + thp_nr_pages(page) - 1; > > I'm pretty sure those two lines should be... > > } else if (folio_test_large(folio)) > pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; > > But, erm ... we're doing this before we have a refcount on the page, > right? So this is unsafe because the page might change which folio > it is in. And the folio we found earlier might become a tail page > of a different folio. (As the comment below explains, HWPoison pages > won't, so it's not unsafe for them). > > Also, thp_nr_pages(page) is going to return 1 for tail pages. So this > is a noop, unless page is a head page. > > It's all a bit confusing, and being memory-hotplug, it's not well > tested. More thought needed. Ehm, it is fairly well tested ;) As memory offlining keeps retrying, temporarily making wrong assumptions about a folio is acceptable, as long as we don't run into BUGs. It's certainly worth a big comment in a code, that this is all racy and that page migration code will stabilize. Now, we could temporarily take a reference, but ... common migration code will try taking its own ref to stabilize the page and would be confused about yet another ref (-> migration will fail). So we have to be careful about grabbing references on these pages, and how long we're going to hold them. Otherwise we'll break memory offlining completely :) -- Thanks, David / dhildenb