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 X-Spam-Level: X-Spam-Status: No, score=-18.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 10F35C43216 for ; Tue, 31 Aug 2021 10:20:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E871460FE8 for ; Tue, 31 Aug 2021 10:20:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240956AbhHaKVR (ORCPT ); Tue, 31 Aug 2021 06:21:17 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:57852 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234682AbhHaKVP (ORCPT ); Tue, 31 Aug 2021 06:21:15 -0400 Received: from imap1.suse-dmz.suse.de (imap1.suse-dmz.suse.de [192.168.254.73]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 7295F221F2; Tue, 31 Aug 2021 10:20:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1630405219; h=from:from:reply-to: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=YpSsXvykYNxP6O/KJXAtzalCJEzC7b1UFSE2Gtv43HY=; b=bKJb7owMtYJrRNv5VDrvvGVp07A/pa7iex0XjJ0+R8PU4XQD6fasNj+V/Plqr/wZYkcmjo NeNsoC7sbZq43MwjOZqsDF97Yf1vaySJQwCgG4nVkrroThrwpt6KhK2BYzUQ/dDxgu0rZC T+sJJ/gtSHHxwveL4jYvw/XxEa1Un2k= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1630405219; h=from:from:reply-to: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=YpSsXvykYNxP6O/KJXAtzalCJEzC7b1UFSE2Gtv43HY=; b=AzUwivJ+XLu5WUD2QTK4k2lJplQkfMSiRyVTG1JT/kB7bRb3QDQ6AMgLI0ror9DP2zzYGZ tp69pYhhuczEMaAA== Received: from imap1.suse-dmz.suse.de (imap1.suse-dmz.suse.de [192.168.254.73]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap1.suse-dmz.suse.de (Postfix) with ESMTPS id 4322F13A7A; Tue, 31 Aug 2021 10:20:19 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap1.suse-dmz.suse.de with ESMTPSA id E/+2D2MCLmE8egAAGKfGzw (envelope-from ); Tue, 31 Aug 2021 10:20:19 +0000 Message-ID: <17a36dfc-cbd1-56c5-46a9-9f4043bb56e6@suse.cz> Date: Tue, 31 Aug 2021 12:20:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0.3 Subject: Re: [PATCH v1 2/2] mm: remove redundant smp_wmb() Content-Language: en-US To: Qi Zheng , akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, david@redhat.com Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com References: <20210828042306.42886-1-zhengqi.arch@bytedance.com> <20210828042306.42886-3-zhengqi.arch@bytedance.com> From: Vlastimil Babka In-Reply-To: <20210828042306.42886-3-zhengqi.arch@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/28/21 06:23, Qi Zheng wrote: > The smp_wmb() which is in the __pte_alloc() is used to > ensure all ptes setup is visible before the pte is made > visible to other CPUs by being put into page tables. We > only need this when the pte is actually populated, so > move it to pte_install(). __pte_alloc_kernel(), It's named pmd_install()? > __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar > to this case. > > We can also defer smp_wmb() to the place where the pmd entry > is really populated by preallocated pte. There are two kinds > of user of preallocated pte, one is filemap & finish_fault(), > another is THP. The former does not need another smp_wmb() > because the smp_wmb() has been done by pte_install(). Same here. > Fortunately, the latter also does not need another smp_wmb() > because there is already a smp_wmb() before populating the > new pte when the THP uses a preallocated pte to split a huge > pmd. > > Signed-off-by: Qi Zheng > Reviewed-by: Muchun Song > --- > mm/memory.c | 47 ++++++++++++++++++++--------------------------- > mm/sparse-vmemmap.c | 2 +- > 2 files changed, 21 insertions(+), 28 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index ef7b1762e996..9c7534187454 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte) > > if (likely(pmd_none(*pmd))) { /* Has another populated it ? */ > mm_inc_nr_ptes(mm); > + /* > + * Ensure all pte setup (eg. pte page lock and page clearing) are > + * visible before the pte is made visible to other CPUs by being > + * put into page tables. > + * > + * The other side of the story is the pointer chasing in the page > + * table walking code (when walking the page table without locking; > + * ie. most of the time). Fortunately, these data accesses consist > + * of a chain of data-dependent loads, meaning most CPUs (alpha > + * being the notable exception) will already guarantee loads are > + * seen in-order. See the alpha page table accessors for the > + * smp_rmb() barriers in page table walking code. > + */ > + smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */ So, could it? :)