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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 61787C2D0C0 for ; Sun, 29 Dec 2019 22:07:10 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E03B9207E0 for ; Sun, 29 Dec 2019 22:07:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E03B9207E0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 4116D8E0005; Sun, 29 Dec 2019 17:07:09 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 3C2958E0003; Sun, 29 Dec 2019 17:07:09 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2D8CF8E0005; Sun, 29 Dec 2019 17:07:09 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0037.hostedemail.com [216.40.44.37]) by kanga.kvack.org (Postfix) with ESMTP id 15E5F8E0003 for ; Sun, 29 Dec 2019 17:07:09 -0500 (EST) Received: from smtpin01.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id B3B53180AD801 for ; Sun, 29 Dec 2019 22:07:08 +0000 (UTC) X-FDA: 76319565336.01.bike62_21304f1f2fe5d X-HE-Tag: bike62_21304f1f2fe5d X-Filterd-Recvd-Size: 4595 Received: from mga18.intel.com (mga18.intel.com [134.134.136.126]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Sun, 29 Dec 2019 22:07:07 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 29 Dec 2019 14:07:06 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,372,1571727600"; d="scan'208";a="243706077" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by fmsmga004.fm.intel.com with ESMTP; 29 Dec 2019 14:07:04 -0800 Date: Mon, 30 Dec 2019 06:07:05 +0800 From: Wei Yang To: Matthew Wilcox Cc: Wei Yang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com Subject: Re: [Patch v2] mm/rmap.c: split huge pmd when it really is Message-ID: <20191229220705.GA22258@richard> Reply-To: Wei Yang References: <20191223222856.7189-1-richardw.yang@linux.intel.com> <20191223231120.GA31820@bombadil.infradead.org> <20191224015602.GB7739@richard> <20191227151346.GA10799@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191227151346.GA10799@bombadil.infradead.org> User-Agent: Mutt/1.9.4 (2018-02-28) 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 Fri, Dec 27, 2019 at 07:13:46AM -0800, Matthew Wilcox wrote: >On Tue, Dec 24, 2019 at 09:56:02AM +0800, Wei Yang wrote: >> On Mon, Dec 23, 2019 at 03:11:20PM -0800, Matthew Wilcox wrote: >> >On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote: >> >> When page is not NULL, function is called by try_to_unmap_one() with >> >> TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one() >> >> with TTU_SPLIT_HUGE_PMD set: >> >> >> >> * unmap_page() >> >> * shrink_page_list() >> >> >> >> In both case, the page passed to try_to_unmap_one() is PageHead() of the >> >> THP. If this page's mapping address in process is not HPAGE_PMD_SIZE >> >> aligned, this means the THP is not mapped as PMD THP in this process. >> >> This could happen when we do mremap() a PMD size range to an un-aligned >> >> address. >> >> >> >> Currently, this case is handled by following check in __split_huge_pmd() >> >> luckily. >> >> >> >> page != pmd_page(*pmd) >> >> >> >> This patch checks the address to skip some work. >> > >> >The description here is confusing to me. >> > >> >> Sorry for the confusion. >> >> Below is my understanding, if not correct or proper, just let me know :-) >> >> According to current comment in __split_huge_pmd(), we check pmd_page with >> page for migration case. While actually, this check also helps in the >> following two cases when page already split-ed: >> >> * page just split-ed in place >> * page split-ed and moved to non-PMD aligned address >> >> In both cases, pmd_page() is pointing to the PTE level page table. That's why >> we don't split one already split-ed THP page. >> >> If current code really intend to cover these two cases, sorry for my poor >> understanding. >> >> >> + /* >> >> + * When page is not NULL, function is called by try_to_unmap_one() >> >> + * with TTU_SPLIT_HUGE_PMD set. There are two places set >> >> + * TTU_SPLIT_HUGE_PMD >> >> + * >> >> + * unmap_page() >> >> + * shrink_page_list() >> >> + * >> >> + * In both cases, the "page" here is the PageHead() of a THP. >> >> + * >> >> + * If the page is not a PMD mapped huge page, e.g. after mremap(), it >> >> + * is not necessary to split it. >> >> + */ >> >> + if (page && !IS_ALIGNED(address, HPAGE_PMD_SIZE)) >> >> + return; >> > >> >Repeating 75% of it as comments doesn't make it any less confusing. And >> >it feels like we're digging a pothole for someone to fall into later. >> >Why not make it make sense ... >> > >> > if (page && !IS_ALIGNED(address, page_size(page)) >> > return; >> >> Hmm... Use HPAGE_PMD_SIZE here wants to emphasize we want the address to be >> PMD aligned. If just use page_size() here, may confuse the audience? > >I'm OK with using HPAGE_PMD_SIZE here. I was trying to future-proof >this function for supporting 64kB pages with a 4kB page size on ARM, >but this function will need changes for that anyway, so I'm OK with >your suggestion. Thanks for your comments. :-) -- Wei Yang Help you, Help me