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 6EB08CA9EA0 for ; Fri, 18 Oct 2019 14:00:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 167AC222BD for ; Fri, 18 Oct 2019 14:00:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 167AC222BD 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 A7A198E0005; Fri, 18 Oct 2019 10:00:17 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A038C8E0003; Fri, 18 Oct 2019 10:00:17 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8F1808E0005; Fri, 18 Oct 2019 10:00:17 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0091.hostedemail.com [216.40.44.91]) by kanga.kvack.org (Postfix) with ESMTP id 6CDBB8E0003 for ; Fri, 18 Oct 2019 10:00:17 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id EC3DF8403 for ; Fri, 18 Oct 2019 14:00:16 +0000 (UTC) X-FDA: 76057064832.18.rice67_1f42685a24030 X-HE-Tag: rice67_1f42685a24030 X-Filterd-Recvd-Size: 5825 Received: from huawei.com (szxga04-in.huawei.com [45.249.212.190]) by imf41.hostedemail.com (Postfix) with ESMTP for ; Fri, 18 Oct 2019 14:00:14 +0000 (UTC) Received: from DGGEMS413-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 1DB18D29E6AA708DFBF9; Fri, 18 Oct 2019 22:00:06 +0800 (CST) Received: from [127.0.0.1] (10.133.219.218) by DGGEMS413-HUB.china.huawei.com (10.3.19.213) with Microsoft SMTP Server id 14.3.439.0; Fri, 18 Oct 2019 22:00:01 +0800 Message-ID: <5DA9C560.4010009@huawei.com> Date: Fri, 18 Oct 2019 22:00:00 +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: John Hubbard CC: , , 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> In-Reply-To: <60492b3f-c933-b54c-8551-18ebb813d7d1@nvidia.com> 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 2019/10/18 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? Thanks, zhong jiang > 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) { > + if ((ret > 0) && migrate_allow) { > + nr_pages = ret; > drain_allow = true; > goto check_again; > } > } > > - return nr_pages; > + return ret; > } > #else > static long check_and_migrate_cma_pages(struct task_struct *tsk, > > > thanks, > > John Hubbard > NVIDIA > > . >