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=-16.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,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=ham 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 DBDE4C4320E for ; Tue, 31 Aug 2021 12:52:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BC4D561041 for ; Tue, 31 Aug 2021 12:52:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231917AbhHaMxs (ORCPT ); Tue, 31 Aug 2021 08:53:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42654 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232251AbhHaMxq (ORCPT ); Tue, 31 Aug 2021 08:53:46 -0400 Received: from mail-pj1-x102b.google.com (mail-pj1-x102b.google.com [IPv6:2607:f8b0:4864:20::102b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D6686C06175F for ; Tue, 31 Aug 2021 05:52:51 -0700 (PDT) Received: by mail-pj1-x102b.google.com with SMTP id mw10-20020a17090b4d0a00b0017b59213831so1950131pjb.0 for ; Tue, 31 Aug 2021 05:52:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding; bh=WabTdj9An5z5ILEGVeAkv4WhYJoKZFzGY2Bi+4jqu40=; b=QtbFCBQwB7vEOOjsNeuQm4+4/dYAfMe3yCq4qGgDMPEP2HtgL0c0UNl/2IPOdesOwR HDN5aoeyLSKvd+pOu/bNSqj8hw6KjAf4fMtcvarHbALjsd0qhaiAcfKjFl/s6XmzbDuw vLl3jBaDIzoMLuMw5BwFwqtLcpVMUyTzUszMSRvVcSFXLn4/4JEu/EVetR9Z6cTOAjY5 cdOVD9RO944Bt20RgMPPgjxMYbMVnck3LxIogxCXaDIWuG4hPo5ECGXyanqvTrD7gEgS X9pMjDGgKSHs87IlIUdATLZY0ETAiGXiePf2ln3VFdMx1NnXGtSG8wHIgXcvGnketO+E hRbg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding; bh=WabTdj9An5z5ILEGVeAkv4WhYJoKZFzGY2Bi+4jqu40=; b=qd+GFUUYFCXycviCO4ZAPHXtuHEKV3Pf2IMVr+IvnoTpS/oXJPc8PHFk4Sca0VFXCV gcQ+Yq5qfJ6+FGXenhxo89khvud/TB2xmyECxBT/F6E5MTNS0raaVRGIve7YjCXNWBl1 UCN3yyYXHTKZ0/+dzmFJFGJZoukZZEkFk1NhqeVHKm26olKUveAMTTAYeIrRamnMY+TO 41gTifMOJpz7S45nrlBPhWn1ZTxIuzwWdUWnO5Go3Jn/M5SFp00yk6gdFRpIkaOxkeWo W6gAjf2zIrkMhVYFeM0znjtMnVBNjhXu68TTzcq2CGb+oE9NVW9+xJB7lvYz5PirsX5F N/Qw== X-Gm-Message-State: AOAM531IHvM8OwYdpeH4IYtN0042aiwFkNMkGtlrm8u91cL/QqMH5iyg VRu3yFtG0ze/VQ00nz7jOxdCSg== X-Google-Smtp-Source: ABdhPJzUHSDAap4VmW6faAIG74gQrS2lCsqGRTOMEhdKrHauFD9+QCaCpWYcdIqu3MWTwEcslg6jEA== X-Received: by 2002:a17:902:c643:b0:138:b603:f944 with SMTP id s3-20020a170902c64300b00138b603f944mr4593654pls.45.1630414371312; Tue, 31 Aug 2021 05:52:51 -0700 (PDT) Received: from [10.254.46.235] ([139.177.225.230]) by smtp.gmail.com with ESMTPSA id s29sm20965758pgl.38.2021.08.31.05.52.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Aug 2021 05:52:50 -0700 (PDT) Subject: Re: [PATCH v1 2/2] mm: remove redundant smp_wmb() To: Vlastimil Babka , 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> <17a36dfc-cbd1-56c5-46a9-9f4043bb56e6@suse.cz> From: Qi Zheng Message-ID: <766d0039-02c9-4511-6421-9d2ccfd0cfb2@bytedance.com> Date: Tue, 31 Aug 2021 20:52:42 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <17a36dfc-cbd1-56c5-46a9-9f4043bb56e6@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/8/31 PM6:20, Vlastimil Babka wrote: > 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()? Yes, I will update it in the next version. > >> __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? :) > Yes, it could, but we don't have smp_wmb__after_spin_lock() now.