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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 8D000CA9EAF for ; Mon, 21 Oct 2019 15:11:57 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id F325F2084B for ; Mon, 21 Oct 2019 15:11:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F325F2084B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=huawei.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 95BB46B0003; Mon, 21 Oct 2019 11:11:56 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 90BEF6B0005; Mon, 21 Oct 2019 11:11:56 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 822566B0006; Mon, 21 Oct 2019 11:11:56 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0163.hostedemail.com [216.40.44.163]) by kanga.kvack.org (Postfix) with ESMTP id 5F9746B0003 for ; Mon, 21 Oct 2019 11:11:56 -0400 (EDT) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id EB12D45BC for ; Mon, 21 Oct 2019 15:11:55 +0000 (UTC) X-FDA: 76068131790.08.chain59_66d1320379213 X-HE-Tag: chain59_66d1320379213 X-Filterd-Recvd-Size: 5928 Received: from huawei.com (szxga07-in.huawei.com [45.249.212.35]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Mon, 21 Oct 2019 15:11:46 +0000 (UTC) Received: from DGGEMS402-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 93CDE522D4DCEF32D051; Mon, 21 Oct 2019 23:11:07 +0800 (CST) Received: from [127.0.0.1] (10.133.219.218) by DGGEMS402-HUB.china.huawei.com (10.3.19.202) with Microsoft SMTP Server id 14.3.439.0; Mon, 21 Oct 2019 23:11:02 +0800 Message-ID: <5DADCA85.3080209@huawei.com> Date: Mon, 21 Oct 2019 23:11:01 +0800 From: zhong jiang User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Alexander Duyck CC: John Hubbard , Andrew Morton , Vlastimil Babka , linux-mm Subject: Re: [PATCH] mm: Unsigned 'nr_pages' always larger than zero References: <1571282386-65076-1-git-send-email-zhongjiang@huawei.com> <60492b3f-c933-b54c-8551-18ebb813d7d1@nvidia.com> <5DA9C560.4010009@huawei.com> In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.133.219.218] X-CFilter-Loop: Reflected 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 2019/10/19 0:07, Alexander Duyck wrote: > On Fri, Oct 18, 2019 at 7:00 AM zhong jiang wrote: >> On 2019/10/18 4:42, John Hubbard wrote: >>> On 10/16/19 8:19 PM, zhong jiang wrote: >>>> With the help of unsigned_lesser_than_zero.cocci. Unsigned 'nr_pages"' >>>> compare with zero. And __get_user_pages_locked will return an long value. >>>> >>>> The patch use a new local variable to store the return value of >>>> __get_user_pages_locked(). Then use it to compare with zero. >>> Hi Zhong, >>> >>> The above are actually more like notes to yourself, and while those are >>> good to have, but it's not yet a perfect commit description: it talks >>> about how you got here, which is only part of the story. A higher level >>> summary is better, and let the code itself cover the details. >>> >>> Like this: >>> >>> First line (subject): >>> >>> mm/gup: allow CMA migration to propagate errors back to caller >>> >>> Commit description: >>> >>> check_and_migrate_cma_pages() was recording the result of >>> __get_user_pages_locked() in an unsigned "nr_pages" variable. Because >>> __get_user_pages_locked() returns a signed value that can include >>> negative errno values, this had the effect of hiding errors. >>> >>> Change check_and_migrate_cma_pages() implementation so that it >>> uses a signed variable instead, and propagates the results back >>> to the caller just as other gup internal functions do. >>> >>> This was discovered with the help of unsigned_lesser_than_zero.cocci. >>> >>>> Suggested-by: Andrew Morton >>>> Signed-off-by: zhong jiang >>>> --- >>>> mm/gup.c | 8 +++++--- >>>> 1 file changed, 5 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/mm/gup.c b/mm/gup.c >>>> index 8f236a3..1fe0ceb 100644 >>>> --- a/mm/gup.c >>>> +++ b/mm/gup.c >>>> @@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk, >>>> bool drain_allow = true; >>>> bool migrate_allow = true; >>>> LIST_HEAD(cma_page_list); >>>> + long ret; >>> Ira pointed out that this needs initialization, see below. >>> >>>> check_again: >>>> for (i = 0; i < nr_pages;) { >>>> @@ -1504,17 +1505,18 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk, >>>> * again migrating any new CMA pages which we failed to isolate >>>> * earlier. >>>> */ >>>> - nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages, >>>> + ret = __get_user_pages_locked(tsk, mm, start, nr_pages, >>>> pages, vmas, NULL, >>>> gup_flags); >>>> >>>> - if ((nr_pages > 0) && migrate_allow) { >>>> + nr_pages = ret; >>> Although technically correct, it makes me feel odd to see the assignment >>> done from signed to unsigned, *before* checking for >0. And Ira is hinting >>> at the same thing, when he asks if we can return early here. See below... >>> >>>> + if ((ret > 0) && migrate_allow) { >>>> drain_allow = true; >>>> goto check_again; >>>> } >>>> } >>>> >>>> - return nr_pages; >>>> + return ret; >>>> } >>>> #else >>>> static long check_and_migrate_cma_pages(struct task_struct *tsk, >>>> >>> So, overall, I'd recommend this, instead: >>> >>> diff --git a/mm/gup.c b/mm/gup.c >>> index 23a9f9c9d377..72bc027037fa 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -1443,6 +1443,7 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk, >>> bool drain_allow = true; >>> bool migrate_allow = true; >>> LIST_HEAD(cma_page_list); >>> + long ret = nr_pages; >>> >> I think the ret should be assigned as zero. we will return ret if cma_page_list is empty. >> I miss something? > That wasn't the behavior before. Before if the cma_page_list was empty > nr_pages would not be modified from what what passed. I believe that > is the intended behavior for this since the cma_page_list seems like a > workaround to migrate out any CMA pages if present in the longterm > mapping. If there are no CMA pages the return result should be > nr_pages. Thanks for your clarification. Sincerely, zhong jiang > Thanks. > > - Alex > > > . >