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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 19C99C4167D for ; Mon, 9 May 2022 08:57:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237314AbiEII6R (ORCPT ); Mon, 9 May 2022 04:58:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51732 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236759AbiEIItl (ORCPT ); Mon, 9 May 2022 04:49:41 -0400 Received: from out30-54.freemail.mail.aliyun.com (out30-54.freemail.mail.aliyun.com [115.124.30.54]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9973517B60D; Mon, 9 May 2022 01:45:46 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R501e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=30;SR=0;TI=SMTPD_---0VChb3kD_1652085922; Received: from 30.32.96.14(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VChb3kD_1652085922) by smtp.aliyun-inc.com(127.0.0.1); Mon, 09 May 2022 16:45:24 +0800 Message-ID: Date: Mon, 9 May 2022 16:46:03 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.8.1 Subject: Re: [PATCH v2 1/3] mm: change huge_ptep_clear_flush() to return the original pte To: Christophe Leroy , Muchun Song Cc: "dalias@libc.org" , "linux-ia64@vger.kernel.org" , "linux-sh@vger.kernel.org" , "linux-mips@vger.kernel.org" , "James.Bottomley@HansenPartnership.com" , "linux-mm@kvack.org" , "paulus@samba.org" , "sparclinux@vger.kernel.org" , "agordeev@linux.ibm.com" , "will@kernel.org" , "linux-arch@vger.kernel.org" , "linux-s390@vger.kernel.org" , "arnd@arndb.de" , "ysato@users.sourceforge.jp" , "deller@gmx.de" , "catalin.marinas@arm.com" , "borntraeger@linux.ibm.com" , "gor@linux.ibm.com" , "hca@linux.ibm.com" , "linux-arm-kernel@lists.infradead.org" , "tsbogend@alpha.franken.de" , "linux-parisc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "svens@linux.ibm.com" , "akpm@linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" , "davem@davemloft.net" , "mike.kravetz@oracle.com" References: <012a484019e7ad77c39deab0af52a6755d8438c8.1652002221.git.baolin.wang@linux.alibaba.com> From: Baolin Wang In-Reply-To: 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 5/9/2022 1:46 PM, Christophe Leroy wrote: > > > Le 08/05/2022 à 15:09, Baolin Wang a écrit : >> >> >> On 5/8/2022 7:09 PM, Muchun Song wrote: >>> On Sun, May 08, 2022 at 05:36:39PM +0800, Baolin Wang wrote: >>>> It is incorrect to use ptep_clear_flush() to nuke a hugetlb page >>>> table when unmapping or migrating a hugetlb page, and will change >>>> to use huge_ptep_clear_flush() instead in the following patches. >>>> >>>> So this is a preparation patch, which changes the >>>> huge_ptep_clear_flush() >>>> to return the original pte to help to nuke a hugetlb page table. >>>> >>>> Signed-off-by: Baolin Wang >>>> Acked-by: Mike Kravetz >>> >>> Reviewed-by: Muchun Song >> >> Thanks for reviewing. >> >>> >>> But one nit below: >>> >>> [...] >>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>> index 8605d7e..61a21af 100644 >>>> --- a/mm/hugetlb.c >>>> +++ b/mm/hugetlb.c >>>> @@ -5342,7 +5342,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct >>>> *mm, struct vm_area_struct *vma, >>>>           ClearHPageRestoreReserve(new_page); >>>>           /* Break COW or unshare */ >>>> -        huge_ptep_clear_flush(vma, haddr, ptep); >>>> +        (void)huge_ptep_clear_flush(vma, haddr, ptep); >>> >>> Why add a "(void)" here? Is there any warning if no "(void)"? >>> IIUC, I think we can remove this, right? >> >> I did not meet any warning without the casting, but this is per Mike's >> comment[1] to make the code consistent with other functions casting to >> void type explicitly in hugetlb.c file. >> >> [1] >> https://lore.kernel.org/all/495c4ebe-a5b4-afb6-4cb0-956c1b18d0cc@oracle.com/ >> > > As far as I understand, Mike said that you should be accompagnied with a > big fat comment explaining why we ignore the returned value from > huge_ptep_clear_flush(). > > By the way huge_ptep_clear_flush() is not declared 'must_check' so this > cast is just visual polution and should be removed. > > In the meantime the comment suggested by Mike should be added instead. Sorry for my misunderstanding. I just follow the explicit void casting like other places in hugetlb.c file. And I am not sure if it is useful adding some comments like below, since we did not need the original pte value in the COW case mapping with a new page, and the code is more readable already I think. Mike, could you help to clarify what useful comments would you like? and remove the explicit void casting? Thanks. /* * Just ignore the return value with new page mapped. */