Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <7239e60d-9873-52b7-2e05-43d5500d6459@nvidia.com>
Date:   Thu, 3 Feb 2022 13:27:17 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.5.1
Subject: Re: [PATCH v3 4/4] mm/gup: remove get_user_pages_locked()
Content-Language: en-US
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        David Hildenbrand <david@redhat.com>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        Jan Kara <jack@suse.cz>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Jason Gunthorpe <jgg@nvidia.com>
References: <20220203093232.572380-1-jhubbard@nvidia.com>
 <20220203093232.572380-5-jhubbard@nvidia.com>
 <YfvgQHewEbN3lR33@infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <YfvgQHewEbN3lR33@infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?dnJIaXhscmlBZUZ6b05CVVovQUZ1SktZcDc2V2txeTBkWGMvU1czY3BWb1Uv?=
 =?utf-8?B?QlowVEw3Mm1vSFNyUGpNVlo0dUxYMkh2YVFFdXFjL1FHZ2d5TUFpb2FPLzNB?=
 =?utf-8?B?bUxBUlVackM0czlFTGRQcHJwdEJoaTZhNlB1eEtqWSt1VENad2phWGFNa0Zu?=
 =?utf-8?B?b0dQb1RqVUFzWWNwZkkzSUVSc0FjVkFpaGM1L2N2bkRQblBTMlpVYkNjRVZy?=
 =?utf-8?B?Z2p0bGxPS2ltclBrZkt0QUlXVUl0ajB2VnJjZ2lMVGZGYUVOZUlqMHpEVEVV?=
 =?utf-8?B?VFVZSUlCRUlKRmtmY2h3SkRhZDBCRWZHcHlrM0ZXSjVOb3dvdVFjOUl0cHhC?=
 =?utf-8?B?ZEtsZW5KZnZxQnBOZ3JRa2w2WDNOM2FJTVMxMjRlN2JPeUtIVCtUMHB5Um9o?=
 =?utf-8?B?SXEyeEcwSnJGU1BWTGxmZThLSEVBcnJKMFlWR3dCTFduTzlOOE5qZUQ4ZzYy?=
 =?utf-8?B?dmhLTncrQ2l3cDd6NVVqTFV6bk9LK3RBeGtBeEliSFcxamQwSll0MG94QTkw?=
 =?utf-8?B?Y2pSNG41WXdreU1ob2hCcEpZMXBjRGpiR2N5N1ExZy9RY2ZkNCtuRWVzQWRN?=
 =?utf-8?B?NTlKMFRSRVRwamF2LzdZME9TZXd2c28xd2hGZk1ZTUNObkw3ZEJqeUpISDM0?=
 =?utf-8?B?djhEZlBRdnhoTk0xallYaDNBc0drVk12TGdEbTRjZkNURHo1MGdDeUpOcmU1?=
 =?utf-8?B?LzN4SjNqZmd6dzlhY0hndE1tdnJVMnY3a3hWUjM5YXp5cHhjYWwydHFBVDdu?=
 =?utf-8?B?R3BWb1Q1UEs0bTBhdVU1cFNEK3Uzd2k4SWRqeFhNc1BVRDRwblZvc1NxV09W?=
 =?utf-8?B?RlpZK0ZLeEhnd0E5S1NZT1hvWjZ0TTR4T0cybndHZUxpbjUxVDB1Qk5mYTJZ?=
 =?utf-8?B?L0ZLNTN0Yi9GNDJkWVlSTkdyVVJwdnVSWEY5MHExSHNEQmxxUmZnVTAxQ1J1?=
 =?utf-8?B?MGxkenRGSGIyWTNVblBRbWRmYkZsWkFCWHRoeTIwdWIxZk4ybXhWcS9LdnVV?=
 =?utf-8?B?eFEzRWxPR2VMMXo1NmpoMVpvUHpuVkZZejJYQ3llUFlxbFl5TXV1SjEwVjky?=
 =?utf-8?B?VWhjSU5qWWJITTdMSlFUaUgwMS91RzZGMTNrOG11L0RYL2kvaW1aeGxDMHJV?=
 =?utf-8?B?SjdWbFgwNGJmN2ExMDNVSXN4K0NYUkxsZnJ0MmZSaU5CcVpTeE45T1YyMFY2?=
 =?utf-8?B?MlFwQXJsVlI1K3FTRGZQTVJkd3MxbjVOZzY4MWRMYkZOWUxnY2EzVnlrcDdM?=
 =?utf-8?B?NjlqWXJLbW9xL1orUkVqZ0k2SjU5K2dtbnoxcGNHVkpKL3BrQllFUDVPT0c5?=
 =?utf-8?B?QWFQSUJDY1k1WG8zdTFFV0NNa1hlakZCYzJpaTJpeWFwY0tTN2Jrbis0WkJw?=
 =?utf-8?B?c01BVVpSSHpBWTdSZjNhdlQzZDdiYzRGaGhoUlN1UGVhUndzYXV1SEhGV3or?=
 =?utf-8?B?YzREcTBPWW5wQjBOVmk5L0M5azlKd2NEcDkvWVltVmNLemxFbnNTV2pTYUhU?=
 =?utf-8?B?bERTdGVOeElCZ3JQSzhkWlRtVWpHREdPVHYzeG1sV0t5a1kxcUV3UW5oelVw?=
 =?utf-8?B?aTlLZHoxZitQY0lUZ3o2TEZXbnFaRERiUUNucU5wS1VEMk5QelVSUjdXTWRs?=
 =?utf-8?B?bDc2TDRWNTkyYVpaaWFkV0VVMGZFV0Y0dHNSNGxkaEgvb2JVUlB3U3F0UEFz?=
 =?utf-8?B?bmpaby9CYXRYMFhFTnVhVVdGTWt1NjVpT2JPQXVkZlBYZ3dValZ5S0RUcTI4?=
 =?utf-8?B?eklXemkwMnZESWwxQThFSng2ZXhXV3BabWZWTVhYOHo0S2Z5TS9zcVk4N0JL?=
 =?utf-8?B?RWlIWlZjZkdQRDBuU2oxZ0Q1S0svTDJ6a1U2cFU1K0lPSW5rVGd6UDFpRXNY?=
 =?utf-8?B?dkFyUnB4RGljTm1hYndmU0gvaTZuQzcwMU1nMWtyUmg4THJmMCtvNWR1OEFY?=
 =?utf-8?B?OVdhVGJWY3BaWDR0MC8xS2ZlaXJGMDU0b0swN0lFM3dVb0hJUmVIMWE5R01J?=
 =?utf-8?B?YURSR045bDR2WENibENOTEtjcFE0WkZpWXM1VE9uamRnSGZSYXdNNDZZcWd4?=
 =?utf-8?B?SVlBdkxrRU1menNkWndTdDhKckNZbmg5M1pGc2tNb090cThVbWg0UDJHUita?=
 =?utf-8?B?SUZRWlJsMDBhYWkzeTBqUnJYVU83SHg5M3VsZlFLUUxDanBSbkwyNXVPemdB?=
 =?utf-8?Q?5AgAxopQB1RwvA6zMnexnto=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 129405b1-5fc4-4561-0f1f-08d9e75bf547
X-MS-Exchange-CrossTenant-AuthSource: BY5PR12MB4130.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Feb 2022 21:27:19.4156
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: ite58sibcaoTEBzmjVozn5zwQG8Dk8iQAsfb4w8Z/UsDh/JO5o7sYhqGo0Zry7bTXxSCOpi3WNh933xLpsnsYA==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR1201MB2488
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 2/3/22 06:01, Christoph Hellwig wrote:
> On Thu, Feb 03, 2022 at 01:32:32AM -0800, John Hubbard wrote:
>> There is only one caller of get_user_pages_locked(). The purpose of
>> get_user_pages_locked() is to allow for unlocking the mmap_lock when
>> reading a page from the disk during a page fault (hidden behind
>> VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used
>> mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in
>> fact I've used some of his wording here.)
>>
>> However, it is unlikely for lookup_node() to take a page fault. With
>> that in mind, change over to calling get_user_pages_fast(). This
>> simplifies the code, runs a little faster in the expected case, and
>> allows removing get_user_pages_locked() entirely.
> 
> Maybe split the lookup_node changes into a separate patch, as that
> allows to document that change even better.

OK, I'll do that.


thanks,
-- 
John Hubbard
NVIDIA