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 5F42FECAAD1 for ; Wed, 31 Aug 2022 19:09:03 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id BA3648D0001; Wed, 31 Aug 2022 15:09:02 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B52266B0073; Wed, 31 Aug 2022 15:09:02 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A41408D0001; Wed, 31 Aug 2022 15:09:02 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 90FA36B0072 for ; Wed, 31 Aug 2022 15:09:02 -0400 (EDT) Received: from smtpin05.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 6103880231 for ; Wed, 31 Aug 2022 19:09:02 +0000 (UTC) X-FDA: 79860825324.05.9243F3C Received: from mail-pj1-f45.google.com (mail-pj1-f45.google.com [209.85.216.45]) by imf09.hostedemail.com (Postfix) with ESMTP id 28200140053 for ; Wed, 31 Aug 2022 19:09:01 +0000 (UTC) Received: by mail-pj1-f45.google.com with SMTP id h13-20020a17090a648d00b001fdb9003787so205352pjj.4 for ; Wed, 31 Aug 2022 12:09:01 -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=ji1g99B3ksPEqP/vBsoq8oWjl/Stw3yVojPXqCh1rP0=; b=X6LjA+o1ewCZZiobat06cWgAPq+M0EmmBVSqk68O97COSS3nwBlV/8WsDywUQvdpgQ LeMOOuEOUhuZJ0qLm10zaV2AwUD6Iy/eRMNz8kCAQLpo33Lrnk9tmISNehXijggetaVY 0rCiJLjOGFo9hdx+jNZ8oz7E6S79Lm7cZY+ATGKKzmeVlFM5Ki7FWS9DpbjONlcNhK3s CretUBiw+OFk84Kgo5c58ztcXdH0lvEJEq0QoJRjoPdSr88ckg2ul3q7s5nC04ZRQPat TpMKkzV85lX3K8XuIvhPJWJVkf2Bv4Q1B+NOiRYjFoT6U0iiFHBfAQJNsbsSsnu/cOO2 jlIg== 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=ji1g99B3ksPEqP/vBsoq8oWjl/Stw3yVojPXqCh1rP0=; b=sTvWv5QBnDv1ga4Nl0RQC3chWm1Z2yNY5VX9Je5IGcQyhbbSS4kpRnvRnsEsh6o7PF VuM2fQrXJKNbWAJov0Bhjqadg0bd7kGAPbuWfXZ23IXci7A9kosoVPDJBY7dcfIsGQaQ 9hecobonkW8+k/buDrkaKKVHKMFWYM+k77k8ZfPFpgrpt2uqUP9LdrOQcXrYZ/VH8Ddm y0ZwpyLuwAJTFkOk1FZ2u4K/aXdRTzXMbmveE7l+y5yq18EYpVfCyJrxrqYlAK5VBINh iK3Y7EZDXQxBFDokb/ZfpYt6gsmiAp6Q6khR9jv/nuY9HDpyuWFGCO0rzD5YOKbZxhwv 9vqA== X-Gm-Message-State: ACgBeo2SK8kxxX0f5ajcQlH+azy2vEd3aqOqplTbTaqotOzc8IH7G2Yp VbsY6npoMaiZwcCuibnD+2V4yW9wPUfvIXfQcag= X-Google-Smtp-Source: AA6agR4KWusGvy70OzqRV+jD6Ni977koXVjdSM/62qROg0vxeWZQT/2eUEEwwxglrjOdyiKz+e4axEAVHnOhLKgDQPg= X-Received: by 2002:a17:90a:6b0d:b0:1fa:c6fe:db6 with SMTP id v13-20020a17090a6b0d00b001fac6fe0db6mr4665036pjj.99.1661972941031; Wed, 31 Aug 2022 12:09:01 -0700 (PDT) MIME-Version: 1.0 References: <20220831083024.37138-1-david@redhat.com> <4845ae71-b7dd-1707-ebc3-2eb3521e7fa0@redhat.com> In-Reply-To: <4845ae71-b7dd-1707-ebc3-2eb3521e7fa0@redhat.com> From: Yang Shi Date: Wed, 31 Aug 2022 12:08:48 -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-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661972942; a=rsa-sha256; cv=none; b=ExRpKJGVTHisCkydXn2lDpaMLikjpmsBBOZgMwvRibcbmobtrDrDDBT5Oj6usffprHU2s8 KML2A8Jm8vIKQQOpRP+BxnFwOPUGl0G7F/iMYIvhayn/xiDw8/SJRK5MlxhDvAVVGT95oH OS4XXSaVe0o5ZZ2uRi+bjwRMtuSEhoM= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=X6LjA+o1; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf09.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.45 as permitted sender) smtp.mailfrom=shy828301@gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661972942; 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=ji1g99B3ksPEqP/vBsoq8oWjl/Stw3yVojPXqCh1rP0=; b=YmYEqETTrVwe50Pw62d0WDpSzjPehR2fW2zDLLA2f1AxNY6Hdf4iMkCNJtMxPH87Oc0eO1 daim+jGkSfa44lRgG+RrtnTH2BVq1EyNwgJbey9MUVacH7Mln1zr2CkNQiAjq3bwECdKcl vOTGK60Pis2yE5GhlKWhMBF8+Tl75ks= X-Rspam-User: Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=X6LjA+o1; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf09.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.45 as permitted sender) smtp.mailfrom=shy828301@gmail.com X-Rspamd-Server: rspam07 X-Stat-Signature: noyzu3whsr6hfq55tmh7ardcis695hbr X-Rspamd-Queue-Id: 28200140053 X-HE-Tag: 1661972941-758284 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 11:29 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. > > -- > Thanks, > > David / dhildenb >