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 64D88ECAAD1 for ; Wed, 31 Aug 2022 20:38:38 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id BEC046B0072; Wed, 31 Aug 2022 16:38:37 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B73E66B0073; Wed, 31 Aug 2022 16:38:37 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9EDD78D0001; Wed, 31 Aug 2022 16:38:37 -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 88D436B0072 for ; Wed, 31 Aug 2022 16:38:37 -0400 (EDT) Received: from smtpin19.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id AF76AA1046 for ; Wed, 31 Aug 2022 20:38:35 +0000 (UTC) X-FDA: 79861050990.19.C04674D Received: from mail-pg1-f175.google.com (mail-pg1-f175.google.com [209.85.215.175]) by imf18.hostedemail.com (Postfix) with ESMTP id 5C1F71C0045 for ; Wed, 31 Aug 2022 20:38:35 +0000 (UTC) Received: by mail-pg1-f175.google.com with SMTP id b196so4790424pga.7 for ; Wed, 31 Aug 2022 13:38:35 -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=CCmnkqfGKHR/ON3gNrTQzrwetbHYAD+7v2zifG9dM50=; b=RtSaqcWk+ltgPsCb9uVdNp/i5srYHnIIr55HaWe1QYcKqCb3ZUenJNi7OfXzgb6ZrS uBDdvE17sYzjYv3HvRzsTkMTKC03jsXNCMituxFfC9yGWsu+rX69VdkLNn8AH0vLG/QG hI7Hfl6Sd4nHoNbJ7B80GuIVIuMgZL08NAlqGL0nzIuu27ru0eK9W07Yk4ZqSj8yxuJQ ODaRi7yop03Ww3p5f4fo2j4hNWUNPLy3xQwrO37jJNleyCgjRXU0Ny4IA0mIPi0upB4/ GgZ1cydKufj+1ZzggMUC4Gp2jCUqRVr583/uYeOgmm8KU2qygiGuKWWFbb76c+66LjSz Notg== 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=CCmnkqfGKHR/ON3gNrTQzrwetbHYAD+7v2zifG9dM50=; b=IKSo/F0Kn/mAW353Qt8TVYLH/5SUIdXev/F41Gl87JwBzJQzSpOSoJvSPHLY4fhPn0 fYKV8vHT8QUARJCSIvM0OaMeBpJDBSznCevsKoEeJAUQFYnY/6McfIUFS3fDq3Lapsim +WKEdLWFM+VHlaZxmEkOYRG08cu4I+PfaRCWgKoxIibq38H/gTpX5CODFoskHUBDXnLs KEdiWfZEi/plV3G6aDpwFY+RHo2PvT6CMDHQaHnfNLtdB1s/0rTP6QM3E8XZC8bVPO9p D4zDlmwNGySVOpOHfXyOJryN2iTZcWQiDe0sNxJ7uBzPdP+Tn4BzgLBF1Puyhcmg9DCB ffdQ== X-Gm-Message-State: ACgBeo1wrHV90vevXOh/uru1+96Zya/Y/weSAXIzPWNdpzXkTihQf1t/ TWYU1F9qaQ2aeDp5LusASpdRKRHx5H4M4ILw4Ps= X-Google-Smtp-Source: AA6agR4vskFQkjaeTlcg+3ZueXBytfEkdhsxLidFnFNziE3Do3YDUjjJyob7Mf70s+T4y6fKLNbA+i0ljEyj1T3MlmI= X-Received: by 2002:a65:6a05:0:b0:42c:87a0:ea77 with SMTP id m5-20020a656a05000000b0042c87a0ea77mr9238994pgu.75.1661978314393; Wed, 31 Aug 2022 13:38:34 -0700 (PDT) MIME-Version: 1.0 References: <20220831083024.37138-1-david@redhat.com> In-Reply-To: From: Yang Shi Date: Wed, 31 Aug 2022 13:38:21 -0700 Message-ID: Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page() To: Peter Xu Cc: David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Andrea Arcangeli , Hugh Dickins , "Kirill A. Shutemov" Content-Type: text/plain; charset="UTF-8" ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661978315; a=rsa-sha256; cv=none; b=SbY7GVGnoO1O/p5hByKog3NhPqodOSHzvj1lG3ZkjF3sGlWHuEkOCk1lkQWkM7YO73tODc qxNiwXUK3M95m/bRs+Dl9obChRVx2BlxQrSukmuEFZTldaDzp+b3b/iGV98INqbtbTbjlu X2FHI5YDbiiJ3irH2X0x/0ZrpnFwP0Y= ARC-Authentication-Results: i=1; imf18.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=RtSaqcWk; spf=pass (imf18.hostedemail.com: domain of shy828301@gmail.com designates 209.85.215.175 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661978315; 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=CCmnkqfGKHR/ON3gNrTQzrwetbHYAD+7v2zifG9dM50=; b=8Q8LEm4JtI93A9gSHdme9gktu3Un2BdvPg49qQFK0bzeOqNZP9/pULA9ETbyAU/sKLhJoe TinWI7F/cy9+lnbDCtZnB8IzpDkiRp7U3G037Xz19jkPHsgOGajUJLlmP1jWnn5mCLnUJa a9X5nSa7oSymkPwYIyppFnTGBpc1x34= X-Stat-Signature: 6qa4p6ceiwskx6pftqpy99g1nix61cgd X-Rspamd-Queue-Id: 5C1F71C0045 Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=RtSaqcWk; spf=pass (imf18.hostedemail.com: domain of shy828301@gmail.com designates 209.85.215.175 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1661978315-987129 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 11:52 AM Peter Xu wrote: > > On Wed, Aug 31, 2022 at 10:55:43AM -0700, 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 > > Maybe the pmd level tlb flush is still needed, but on pte level it's > optional (where we can rely on fast-gup rechecking on the pte change)? Do you mean in khugepaged? It does TLB flush, but some arches may not use IPI. > > -- > Peter Xu >