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 5B5CCECAAD1 for ; Wed, 31 Aug 2022 19:43:45 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DD2586B0072; Wed, 31 Aug 2022 15:43:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D82086B0073; Wed, 31 Aug 2022 15:43:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C49E06B0074; Wed, 31 Aug 2022 15:43:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id B54376B0072 for ; Wed, 31 Aug 2022 15:43:44 -0400 (EDT) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 8D5BA141495 for ; Wed, 31 Aug 2022 19:43:44 +0000 (UTC) X-FDA: 79860912768.23.E8BE230 Received: from mail-pj1-f49.google.com (mail-pj1-f49.google.com [209.85.216.49]) by imf31.hostedemail.com (Postfix) with ESMTP id 484CE20057 for ; Wed, 31 Aug 2022 19:43:44 +0000 (UTC) Received: by mail-pj1-f49.google.com with SMTP id j9-20020a17090a3e0900b001fd9568b117so289713pjc.3 for ; Wed, 31 Aug 2022 12:43:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=kSSNFSlAukoDMg7TWjFtnlVW5kvsYnq4LkLWWtnNaYE=; b=gUN+DOp9f+JznK2y/E1t6gj7I4RepxZAebIkWUqABncACk/ikxI5f3Z6JBDnt/iQ98 8x88wI0enbkSGzVyMZwNnow0h9BrTcnk3v7b/35NtdgGeKhzUnharrdMQtJcsoMe4bYc qirwm13AxL4p1z5Bq26B88XqpO+2LHlU5Qr6Bym8WR8koWrIMhOGueqeAL8KsOmdXczt 6/0CnwYwhHoue8kcscfRfVBKvz7zJddGTVM/PEXOXWW58pOvivipzCZ1J294+z7KXiNn 9XskRfwckDjXK6LWRH+7W/w1Z3Ova5XiX9PULHaQELx2/qpijFyOddiIte5BJueq7joM pjXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=kSSNFSlAukoDMg7TWjFtnlVW5kvsYnq4LkLWWtnNaYE=; b=rIFi5Jlwt5TuS5RDpEPQjYYIesJ67zQNJQ/xEpgvXBFCYcQCubFg7d4X8lh6tlNZS9 4VqFr4qyiGNYrRGKGS6oUL0Nnb7pPPbxaJlGWfeX+c2G6e63X4rYylGRWCMiiFnwOZBn 0c+QhHP8/QXYal4DtksuDcv7h4E9yHSFrkum+Ergb3T01VGKAYsCn1IbTZKKRCeY7BGF sHGsF3G4OJFd2+c5uybGuO7mwJmDF3e+k8Ooztooi9NvsAMHkV8ESEf9OEpBNhDyuvwW w3U796thDGwPrzQknuL8f4xvcbXZErQYmSq5B2W6+PLA953258ABXiLb5ID/EuhQvVnZ AeLg== X-Gm-Message-State: ACgBeo1+BildwdqdrZE5IJnBwm6uGfZ6LyKL4N3ZeTLwgkzL5+9zqYFe yLRoFIp2aPW2dQ5M41HesUsRWK8du2/1FNbmy8A= X-Google-Smtp-Source: AA6agR5Rb7PUiQP5I33lNAG2HnP5lfmNWILZ3lh3SDLj8o8dNFcA1ugBSYLnI8Jiz1KL/JzcIdSZ0elgwtVUbZAoN+k= X-Received: by 2002:a17:90b:1b49:b0:1fd:dccc:91f7 with SMTP id nv9-20020a17090b1b4900b001fddccc91f7mr4763539pjb.200.1661975023394; Wed, 31 Aug 2022 12:43:43 -0700 (PDT) MIME-Version: 1.0 References: <20220831083024.37138-1-david@redhat.com> <4845ae71-b7dd-1707-ebc3-2eb3521e7fa0@redhat.com> <94c3217d-df73-2b6b-21f0-95baf117c584@redhat.com> In-Reply-To: <94c3217d-df73-2b6b-21f0-95baf117c584@redhat.com> From: Yang Shi Date: Wed, 31 Aug 2022 12:43:31 -0700 Message-ID: Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page() To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Andrea Arcangeli , Hugh Dickins , Peter Xu , "Kirill A. Shutemov" Content-Type: text/plain; charset="UTF-8" ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661975024; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=kSSNFSlAukoDMg7TWjFtnlVW5kvsYnq4LkLWWtnNaYE=; b=yH3P7Sj+mm4FkzGgMRGvhmnPwfiTVMRztlFifnExL7S/5Jd2zw0vLhMeg3KQE8jtbKHUoj 3cpaMiUG0+/EcYqlOFyazVODj6/eTxzQAQ1ySCS3kTcVRoZ1Z2/S1wcQTOX0QlM6iJJ7ib MV+5Nitt+uQUYJFxisS+DSdYWGXvJ2w= ARC-Authentication-Results: i=1; imf31.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=gUN+DOp9; spf=pass (imf31.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.49 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661975024; a=rsa-sha256; cv=none; b=k6lSpJ062BjtplNZfPTc7u5Q8Is5uqjE6oDv0mD23JxlbYDmKLOXfdGlxfWw8rPDIB0pRt hxnrO0nA4FGQf8uJEtpl7EWPFgogpESeDG+VVYvQUaK3iYVC9GWO13tuaiMTnq5oBTdpGw /R8yJpJ/2w3iBfjIoblGqHcN0ockQyY= X-Rspamd-Queue-Id: 484CE20057 Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=gUN+DOp9; spf=pass (imf31.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.49 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspam-User: X-Rspamd-Server: rspam10 X-Stat-Signature: 8pfhugo351adbqtz7atow9657sponnsb X-HE-Tag: 1661975024-919360 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: On Wed, Aug 31, 2022 at 12:36 PM David Hildenbrand wrote: > > On 31.08.22 21:34, Yang Shi wrote: > > On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand wrote: > >> > >> On 31.08.22 21:08, Yang Shi wrote: > >>> On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand wrote: > >>>> > >>>> On 31.08.22 19:55, Yang Shi wrote: > >>>>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand wrote: > >>>>>> > >>>>>> The comment is stale, because a TLB flush is no longer sufficient and > >>>>>> required to synchronize against concurrent GUP-fast. This used to be true > >>>>>> in the past, whereby a TLB flush would have implied an IPI on architectures > >>>>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts > >>>>>> from completing before completing the flush. > >>>>> > >>>>> Hmm... it seems there might be problem for THP collapse IIUC. THP > >>>>> collapse clears and flushes pmd before doing anything on pte and > >>>>> relies on interrupt disable of fast GUP to serialize against fast GUP. > >>>>> But if TLB flush is no longer sufficient, then we may run into the > >>>>> below race IIUC: > >>>>> > >>>>> CPU A CPU B > >>>>> THP collapse fast GUP > >>>>> > >>>>> gup_pmd_range() <-- see valid pmd > >>>>> > >>>>> gup_pte_range() <-- work on pte > >>>>> clear pmd and flush TLB > >>>>> __collapse_huge_page_isolate() > >>>>> isolate page <-- before GUP bump refcount > >>>>> > >>>>> pin the page > >>>>> __collapse_huge_page_copy() > >>>>> copy data to huge page > >>>>> clear pte (don't flush TLB) > >>>>> Install huge pmd for huge page > >>>>> > >>>>> return the obsolete page > >>>> > >>>> Hm, the is_refcount_suitable() check runs while the PTE hasn't been > >>>> cleared yet. And we don't check if the PMD changed once we're in > >>>> gup_pte_range(). > >>> > >>> Yes > >>> > >>>> > >>>> The comment most certainly should be stale as well -- unless there is > >>>> some kind of an implicit IPI broadcast being done. > >>>> > >>>> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an > >>>> IPI broadcast on THP split (which is a rare event), allows one to > >>>> protect a page table walker by merely disabling the interrupts during > >>>> the walk." > >>>> > >>>> I'm not able to quickly locate that IPI broadcast -- maybe there is one > >>>> being done here (in collapse) as well? > >>> > >>> The TLB flush may call IPI. I'm supposed it is arch dependent, right? > >>> Some do use IPI, some may not. > >> > >> Right, and the whole idea of the RCU GUP-fast was to support > >> architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do > >> it -- but maybe it does so for PMDs? > > > > It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC. > > > > So maybe we should implement pmdp_collapse_flush() for those arches to > > issue IPI. > > ... or find another way to detect and handle this in GUP-fast? > > Not sure if, for handling PMDs, it could be sufficient to propagate the > pmdp pointer + value and double check that the values didn't change. Should work too, right before pinning the page. pmdp_collapse_flush() is actually just called by khugepaged, so arch specific implementation should not be a problem and we avoid making gup fast more complicated. > > -- > Thanks, > > David / dhildenb >