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 D0B6EC433EF for ; Mon, 9 May 2022 02:54:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0C5136B0071; Sun, 8 May 2022 22:54:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 074D96B0073; Sun, 8 May 2022 22:54:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E7DC16B0074; Sun, 8 May 2022 22:54:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id D54056B0071 for ; Sun, 8 May 2022 22:54:07 -0400 (EDT) Received: from smtpin14.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 9B2122F7EC for ; Mon, 9 May 2022 02:54:07 +0000 (UTC) X-FDA: 79444685334.14.925B2AD Received: from mail-pl1-f172.google.com (mail-pl1-f172.google.com [209.85.214.172]) by imf01.hostedemail.com (Postfix) with ESMTP id 096074007C for ; Mon, 9 May 2022 02:53:55 +0000 (UTC) Received: by mail-pl1-f172.google.com with SMTP id d22so12579773plr.9 for ; Sun, 08 May 2022 19:54:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=ZTTQaK1MCP0Shiup/o7qs7DSoxAGZcC20JNp/8zusXE=; b=lvu0tqEAYL4lMpXHlUzMzuGrVzNiSN1F7GkTpaXG15DPrHT1CKlKmPTSyxU7h/NV/j kUWsTNFP/67CPMC2uOZi3lJkE0lLp5sDkHR5Vg4ZoLQibwozZlVvVrWAmrcTfYRpX//B IUvdewS9yYA2LSGRsdHGz3Vy9in8L82rvGFwD3Zc8rtXp9G3nFfXed3dmePb2uW74wWS gjRGNoAkLLOEaS+JIVqDdXS8Khp2rr6+h+bGKqkjlK7OSgl+ubpQebutmaP5tiQRi8Kq 5jTLundScFrLcZoaaz36HUq3ybV0gU6aQtsrRhtprHTBJRtzsvzNZ5USFQbLHaHia+ap v4wA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ZTTQaK1MCP0Shiup/o7qs7DSoxAGZcC20JNp/8zusXE=; b=i+XW/589dRdQcqE+Z+Yyvh7EuDo3yv4SVM0Y6WDBHgNnlMdMQvaR75tOeM/NsxI6Gv t15yPwjGxMT4H/1MMhp4rwnxPvCGn0on9O79TRCpSBifH1Y3/MXhRirXBiaI789GR5Tr uWcu54Ns55dxCUDDNbxybeca++ilbDA7PmwTBbnJvDzRlw0P6e5gNRHgcSy6bVhtLoBY cG4Wtox4CTQRTsgYDGNrmxZH/Q6ErHkSsEYKmZLG9UjYiH2WSkiWQP+Q6ohgfJFc+UNm 16DErwZj3E0RGJZrrUY2yHy/7x6cF4kJfpnSRsjfUVLNNJdeoXKyQcSMEtsEyZdwfdOu zBew== X-Gm-Message-State: AOAM530bTF9LAuyoQB37qsxvrziaQwSlgJTBAKt1Gcw0C5sP9uNg4usC aCMYMQw1BLgg5CxFDLfCuWMF1g== X-Google-Smtp-Source: ABdhPJy7zWNcQupxAcYMUNYtQ/R40mo+W7VEUJcoJcHH9ByfyOdtqYbfEKoLlg6WSLCwLEQ+8OB54A== X-Received: by 2002:a17:902:d2c9:b0:15e:b28d:8ad2 with SMTP id n9-20020a170902d2c900b0015eb28d8ad2mr14262241plc.94.1652064845269; Sun, 08 May 2022 19:54:05 -0700 (PDT) Received: from localhost ([139.177.225.250]) by smtp.gmail.com with ESMTPSA id o18-20020a170902e29200b0015e8d4eb258sm5783241plc.162.2022.05.08.19.54.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 08 May 2022 19:54:04 -0700 (PDT) Date: Mon, 9 May 2022 10:54:00 +0800 From: Muchun Song To: Matthew Wilcox Cc: Baolin Wang , catalin.marinas@arm.com, will@kernel.org, arnd@arndb.de, mike.kravetz@oracle.com, akpm@linux-foundation.org, sj@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH 0/3] Introduce new huge_ptep_get_access_flags() interface Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=lvu0tqEA; spf=pass (imf01.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.214.172 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 096074007C X-Stat-Signature: 8fcbdt5tzo5th4dumas779buzgmrcapy X-HE-Tag: 1652064835-870690 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 Sun, May 08, 2022 at 06:08:18PM +0100, Matthew Wilcox wrote: > On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote: > > As Mike pointed out [1], the huge_ptep_get() will only return one specific > > pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which > > will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD > > size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD > > size hugetlb page for those functions that want to check the dirty or > > young flags of a hugetlb page. For example, the gather_hugetlb_stats() will > > get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring > > will also get inaccurate access statistics. > > > > To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get() > > implementation, which will take into account any subpages' dirty or young bits. > > However we should add a new parameter for ARM64 specific huge_ptep_get() to check > > how many continuous PTEs or PMDs in this CONT-PTE/PMD size hugetlb, that means we > > should convert all the places using huge_ptep_get(), meanwhile most places using > > huge_ptep_get() did not care about the dirty or young flags at all. > > > > So instead of changing the prototype of huge_ptep_get(), this patch set introduces > > a new huge_ptep_get_access_flags() interface and define an ARM64 specific implementation, > > that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size > > hugetlb page. And we can only change to use huge_ptep_get_access_flags() for those > > functions that care about the dirty or young flags of a hugetlb page. > > I question whether this is the right approach. I understand that > different hardware implementations have different requirements here, > but at least one that I'm aware of (AMD Zen 2/3) requires that all > PTEs that are part of a contig PTE must have identical A/D bits. Now, > you could say that's irrelevant because it's x86 and we don't currently > support contPTE on x86, but I wouldn't be surprised to see that other > hardware has the same requirement. > > So what if we make that a Linux requirement? Setting a contPTE dirty or > accessed becomes a bit more expensive (although still one/two cachelines, > so not really much more expensive than a single write). Then there's no > need to change the "get" side of things because they're always identical. > > It does mean that we can't take advantage of hardware setting A/D bits, > unless hardware can be persuaded to behave this way. I don't have any > ARM specs in front of me to check. > I have looked at the comments in get_clear_flush() (in arch/arm64/mm/hugetlbpage.c). That says: /* * If HW_AFDBM is enabled, then the HW could turn on * the dirty or accessed bit for any page in the set, * so check them all. */ Unfortunately, the AD bits are not identical in all subpages. Thanks.