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Wed, 7 Apr 2021 00:54:01 +0000 Subject: Re: [PATCH 2/4] mm/hugeltb: simplify the return code of __vma_reservation_common() To: Miaohe Lin , akpm@linux-foundation.org Cc: n-horiguchi@ah.jp.nec.com, hillf.zj@alibaba-inc.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20210402093249.25137-1-linmiaohe@huawei.com> <20210402093249.25137-3-linmiaohe@huawei.com> From: Mike Kravetz Message-ID: Date: Tue, 6 Apr 2021 17:53:59 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210402093249.25137-3-linmiaohe@huawei.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [50.38.35.18] X-ClientProxiedBy: MW4PR04CA0151.namprd04.prod.outlook.com (2603:10b6:303:85::6) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MW4PR04CA0151.namprd04.prod.outlook.com (2603:10b6:303:85::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4020.16 via Frontend Transport; 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identity=mailfrom; envelope-from=""; helo=userp2130.oracle.com; client-ip=156.151.31.86 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1617756856-652998 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 4/2/21 2:32 AM, Miaohe Lin wrote: > It's guaranteed that the vma is associated with a resv_map, i.e. either > VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would > have returned via !resv check above. So ret must be less than 0 in the > 'else' case. Simplify the return code to make this clear. I believe we still neeed that ternary operator in the return statement. Why? There are two basic types of mappings to be concerned with: shared and private. For private mappings, a task can 'own' the mapping as indicated by HPAGE_RESV_OWNER. Or, it may not own the mapping. The most common way to create a non-owner private mapping is to have a task with a private mapping fork. The parent process will have HPAGE_RESV_OWNER set, the child process will not. The idea is that since the child has a COW copy of the mapping it should not consume reservations made by the parent. Only the parent (HPAGE_RESV_OWNER) is allowed to consume the reservations. Hope that makens sense? > > Signed-off-by: Miaohe Lin > --- > mm/hugetlb.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index a03a50b7c410..b7864abded3d 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -2183,7 +2183,7 @@ static long __vma_reservation_common(struct hstate *h, > return 1; > } > else This else also handles the case !HPAGE_RESV_OWNER. In this case, we never want to indicate reservations are available. The ternary makes sure a positive value is never returned. -- Mike Kravetz > - return ret < 0 ? ret : 0; > + return ret; > } > > static long vma_needs_reservation(struct hstate *h, >