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=-2.2 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 34791C2D0C0 for ; Tue, 24 Dec 2019 01:56:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 068E92070E for ; Tue, 24 Dec 2019 01:56:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727066AbfLXB4E (ORCPT ); Mon, 23 Dec 2019 20:56:04 -0500 Received: from mga03.intel.com ([134.134.136.65]:23701 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726976AbfLXB4E (ORCPT ); Mon, 23 Dec 2019 20:56:04 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 23 Dec 2019 17:56:03 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,349,1571727600"; d="scan'208";a="211727785" Received: from richard.sh.intel.com (HELO localhost) ([10.239.159.54]) by orsmga008.jf.intel.com with ESMTP; 23 Dec 2019 17:56:02 -0800 Date: Tue, 24 Dec 2019 09:56:02 +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: <20191224015602.GB7739@richard> Reply-To: Wei Yang References: <20191223222856.7189-1-richardw.yang@linux.intel.com> <20191223231120.GA31820@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191223231120.GA31820@bombadil.infradead.org> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? -- Wei Yang Help you, Help me