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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4A721C433FE for ; Tue, 10 May 2022 01:35:36 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4Ky0wk5gWqz3cHG for ; Tue, 10 May 2022 11:35:34 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=linux.alibaba.com (client-ip=115.124.30.43; helo=out30-43.freemail.mail.aliyun.com; envelope-from=baolin.wang@linux.alibaba.com; receiver=) Received: from out30-43.freemail.mail.aliyun.com (out30-43.freemail.mail.aliyun.com [115.124.30.43]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4Ky0wG6RxNz2yZf for ; Tue, 10 May 2022 11:35:09 +1000 (AEST) X-Alimail-AntiSpam: AC=PASS; BC=-1|-1; BR=01201311R131e4; CH=green; DM=||false|; DS=||; FP=0|-1|-1|-1|0|-1|-1|-1; HT=alimailimapcm10staff010182156082; MF=baolin.wang@linux.alibaba.com; NM=1; PH=DS; RN=30; SR=0; TI=SMTPD_---0VCo8IMN_1652146499; Received: from 30.15.214.13(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VCo8IMN_1652146499) by smtp.aliyun-inc.com(127.0.0.1); Tue, 10 May 2022 09:35:01 +0800 Message-ID: <3c29b307-5af5-41f0-f97c-c9929f616f53@linux.alibaba.com> Date: Tue, 10 May 2022 09:35:40 +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: Mike Kravetz , Christophe Leroy , Muchun Song 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 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "dalias@libc.org" , "linux-ia64@vger.kernel.org" , "linux-sh@vger.kernel.org" , "catalin.marinas@arm.com" , "linux-kernel@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" , "gor@linux.ibm.com" , "ysato@users.sourceforge.jp" , "deller@gmx.de" , "borntraeger@linux.ibm.com" , "arnd@arndb.de" , "hca@linux.ibm.com" , "linux-arm-kernel@lists.infradead.org" , "tsbogend@alpha.franken.de" , "linux-parisc@vger.kernel.org" , "linux-mips@vger.kernel.org" , "svens@linux.ibm.com" , "akpm@linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" , "davem@davemloft.net" Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 5/10/2022 4:02 AM, Mike Kravetz wrote: > On 5/9/22 01:46, Baolin Wang wrote: >> >> >> 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. >> > > Sorry for the confusion. > > In the original commit, it seemed odd to me that the signature of the > function was changing and there was not an associated change to the only > caller of the function. I did suggest casting to void or adding a comment. > As Christophe mentions, the cast to void is not necessary. In addition, > there really isn't a need for a comment as the calling code is not changed. OK. Will drop the casting in next version. > > The original version of the commit without either is actually preferable. > The commit message does say this is a preparation patch and the return > value will be used in later patches. OK. Thanks Mike for making me clear. Also thanks to Muchun and Christophe.