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=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 25926C43214 for ; Tue, 31 Aug 2021 12:37:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 059CE61041 for ; Tue, 31 Aug 2021 12:37:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232118AbhHaMhy (ORCPT ); Tue, 31 Aug 2021 08:37:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231963AbhHaMhx (ORCPT ); Tue, 31 Aug 2021 08:37:53 -0400 Received: from mail-pl1-x629.google.com (mail-pl1-x629.google.com [IPv6:2607:f8b0:4864:20::629]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 663C5C06175F for ; Tue, 31 Aug 2021 05:36:58 -0700 (PDT) Received: by mail-pl1-x629.google.com with SMTP id q3so10529040plx.4 for ; Tue, 31 Aug 2021 05:36:58 -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=v9g9wZbjQum2R8lBMgDtJLNvpHwYm9wDR3lI7i7rcFU=; b=sNkLyHwNNfFF76COqbJqEQo1Ny08kSwqJPx9SQq+TKr5su2z0NRZX1dL1+G8pcKV/T Yfj2qysGhdNIwTILbExSUF6zQ/Udza5KRyCJlbGnjAJ+UU8TKPK8wlGYZloeNkWqUNWG ZRjGJKU8It967TpSJPsvZyke+gorQdLfLqwXLXOGc+ePXVTNBzdfxXPwHlg269jFdRNN w/f0T2pYp9cj+AAdRQX5altSBt7MYK98voUg7ARMNFeYSdSwuGZ5ZUyIF6SzX4ANE7Fv QejU+iNUiMhb+8Akz7SGUxvpFDKDv+t8nR1Nhki5DAmUWAqafUGzW8PaNoqo5NIcYWZb CJpg== 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=v9g9wZbjQum2R8lBMgDtJLNvpHwYm9wDR3lI7i7rcFU=; b=ExbjOdln1xIxHCpF+vP8gi+1VL/qaEOlgIa++fQPn48lsaute5oDCRmmK5Dj9JBg4e WkBKiUGlphcQCiCuFtXlh907TOkAUnlsYDvAenSFtCfbyColI3sWINqnDKq/wE+oGqif 7nkKbKrgUlgrrdiDuSPx5hZnSlj5EG9aZlAOwLy0bnhMv5TSPA/Bpt7uZieak2c4iffT bNu5KYuyskPwH1pREX+AP5zSNsDlAV+z7isP5fRvFbIEHjEe+nAwLdc0umX0YO/2JxW9 17bVZSbwH1yG1eObmuBI2QakrsJvfNbyqeEYxD0XyqUoqtkEoQpJF32sqTNm2WMiJri1 XhXQ== X-Gm-Message-State: AOAM530rLIARSgTPC7qw1R0lnlk4Eu4HRUxY19Mr27mHaSP4QTF9sh+o 65j4kt+ITwWZj1x8sWcdwd+l3eAWAAGe1w== X-Google-Smtp-Source: ABdhPJwE+H6sMqaImTHXCa39myLxG5X+AE/9mNuCpzedptSRGf6Y7h2n8durJreLqgRFhsWUFEsssQ== X-Received: by 2002:a17:90a:14c4:: with SMTP id k62mr5224838pja.126.1630413417784; Tue, 31 Aug 2021 05:36:57 -0700 (PDT) Received: from [10.254.46.235] ([139.177.225.230]) by smtp.gmail.com with ESMTPSA id j9sm21208132pgl.1.2021.08.31.05.36.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Aug 2021 05:36:57 -0700 (PDT) Subject: Re: [PATCH v1 2/2] mm: remove redundant smp_wmb() To: David Hildenbrand , 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 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> <9da807d4-1fcc-72e0-dc9e-91ab9fbeb7c6@redhat.com> From: Qi Zheng Message-ID: <3f8e9805-b90f-7df3-8514-139afa653671@bytedance.com> Date: Tue, 31 Aug 2021 20:36:51 +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: <9da807d4-1fcc-72e0-dc9e-91ab9fbeb7c6@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/8/31 PM6:02, David Hildenbrand wrote: > On 28.08.21 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(), >> __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(). >> 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 */ >>           pmd_populate(mm, pmd, *pte); >>           *pte = NULL; >>       } >> @@ -451,21 +465,6 @@ int __pte_alloc(struct mm_struct *mm, pmd_t *pmd) >>       if (!new) >>           return -ENOMEM; >> -    /* >> -     * 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 */ >> - >>       pmd_install(mm, pmd, &new); >>       if (new) >>           pte_free(mm, new); >> @@ -478,10 +477,9 @@ int __pte_alloc_kernel(pmd_t *pmd) >>       if (!new) >>           return -ENOMEM; >> -    smp_wmb(); /* See comment in __pte_alloc */ >> - >>       spin_lock(&init_mm.page_table_lock); >>       if (likely(pmd_none(*pmd))) {    /* Has another populated it ? */ >> +        smp_wmb(); /* See comment in pmd_install() */ >>           pmd_populate_kernel(&init_mm, pmd, new); >>           new = NULL; >>       } >> @@ -3857,7 +3855,6 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) >>           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); >>           if (!vmf->prealloc_pte) >>               return VM_FAULT_OOM; >> -        smp_wmb(); /* See comment in __pte_alloc() */ >>       } >>       ret = vma->vm_ops->fault(vmf); >> @@ -3919,7 +3916,6 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, >> struct page *page) >>           vmf->prealloc_pte = pte_alloc_one(vma->vm_mm); >>           if (!vmf->prealloc_pte) >>               return VM_FAULT_OOM; >> -        smp_wmb(); /* See comment in __pte_alloc() */ >>       } >>       vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd); >> @@ -4144,7 +4140,6 @@ static vm_fault_t do_fault_around(struct >> vm_fault *vmf) >>           vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm); >>           if (!vmf->prealloc_pte) >>               return VM_FAULT_OOM; >> -        smp_wmb(); /* See comment in __pte_alloc() */ >>       } >>       return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff); >> @@ -4819,13 +4814,13 @@ int __p4d_alloc(struct mm_struct *mm, pgd_t >> *pgd, unsigned long address) >>       if (!new) >>           return -ENOMEM; >> -    smp_wmb(); /* See comment in __pte_alloc */ >> - >>       spin_lock(&mm->page_table_lock); >>       if (pgd_present(*pgd))        /* Another has populated it */ >>           p4d_free(mm, new); >> -    else >> +    else { >> +        smp_wmb(); /* See comment in pmd_install() */ >>           pgd_populate(mm, pgd, new); >> +    } > > Nit: > > if () { > > } else { > > } > > see Documentation/process/coding-style.rst > > "This does not apply if only one branch of a conditional statement is a > single statement; in the latter case use braces in both branches:" Got it. > > > Apart from that, I think this is fine, > > Acked-by: David Hildenbrand > Thanks, Qi