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Tue, 16 Mar 2021 03:07:02 +0000 Subject: Re: [PATCH v2 5/5] mm/hugetlb: avoid calculating fault_mutex_hash in truncate_op case To: Miaohe Lin , akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20210316022758.52993-1-linmiaohe@huawei.com> From: Mike Kravetz Message-ID: Date: Mon, 15 Mar 2021 20:07:00 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210316022758.52993-1-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: MWHPR1201CA0024.namprd12.prod.outlook.com (2603:10b6:301:4a::34) 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 MWHPR1201CA0024.namprd12.prod.outlook.com (2603:10b6:301:4a::34) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3933.32 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: 1615864032-443566 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 3/15/21 7:27 PM, Miaohe Lin wrote: > The fault_mutex hashing overhead can be avoided in truncate_op case > because page faults can not race with truncation in this routine. So > calculate hash for fault_mutex only in !truncate_op case to save some cpu > cycles. > > Reviewed-by: Mike Kravetz > Signed-off-by: Miaohe Lin > Cc: Mike Kravetz > --- > v1->v2: > remove unnecessary initialization for variable hash > collect Reviewed-by tag from Mike Kravetz My apologies for not replying sooner and any misunderstanding from my previous comments. If the compiler is going to produce a warning because the variable is not initialized, then we will need to keep the initialization. Otherwise, this will show up as a build regression. Ideally, there would be a modifier which could be used to tell the compiler the variable will used. I do not know if such a modifier exists. The patch can not produce a new warning. So, if you need to initialize the variable then do it. My Reviewed-by still applies. -- Mike Kravetz > --- > fs/hugetlbfs/inode.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c > index c262566f7c5d..f7ec94bc7337 100644 > --- a/fs/hugetlbfs/inode.c > +++ b/fs/hugetlbfs/inode.c > @@ -485,7 +485,6 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart, > u32 hash; > > index = page->index; > - hash = hugetlb_fault_mutex_hash(mapping, index); > if (!truncate_op) { > /* > * Only need to hold the fault mutex in the > @@ -493,6 +492,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart, > * page faults. Races are not possible in the > * case of truncation. > */ > + hash = hugetlb_fault_mutex_hash(mapping, index); > mutex_lock(&hugetlb_fault_mutex_table[hash]); > } > >