Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <37484a02-9e64-ea20-314a-a65759b138d2@nvidia.com>
Date:   Mon, 31 Jan 2022 12:01:48 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.5.1
Subject: Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()
Content-Language: en-US
To:     Jan Kara <jack@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        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>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
References: <20220131051752.447699-1-jhubbard@nvidia.com>
 <20220131051752.447699-5-jhubbard@nvidia.com>
 <20220131120557.rwde6sphc245pzu7@quack3.lan>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <20220131120557.rwde6sphc245pzu7@quack3.lan>
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?cm1wVEtYN2QzbHo3bzQyU2pXYUJuazBkUlM0V2Q2UEI3N2FmRzNpMm9oQ01H?=
 =?utf-8?B?SWF2dWNHeklMcWNNajUzcGw2aWxYRlBIV05taTNONWZweDd1RzZXcUpvdTlO?=
 =?utf-8?B?dVpmSFlGSjR6dGtpdVRBd1ViOWFjY2Vrb3RkNHU3RmFjUElXT1NITy91VGUz?=
 =?utf-8?B?Vkk5dW0zR2pTTUVuT1ROZlJ4OVhoS04rVjhRMmNDeXYrdFc2SC9lRkdvMmc2?=
 =?utf-8?B?MDVmcE9UdDNNek1vVVJTSGRSLzl2NjlGUGlyQnVFNTltMTZveGR4TDFIa3Fj?=
 =?utf-8?B?ZC9nMkxwY3RtbDJ6dSs1ZlRkRjlmWGFOYm1kUXU0MUg3cFB6c3A4VHNJcW5q?=
 =?utf-8?B?UnZGYkduMmNUb0JUYXduaW5Vb3RVNTM1RmFiZVhibGNOK1hVMDJOVXlOTi83?=
 =?utf-8?B?eXkrQ0lMYzZCMG9kL0s1S0lwcEhJRjVObStwa3lLaWJQaGJWellCWHZYZ2tv?=
 =?utf-8?B?a0krQ2liMFR6ejV3Uzhua3JvODNpZEdod2FhSmdrOVN0TUordHppZzVha3h2?=
 =?utf-8?B?SjlDUzROallodHBmVVJ6SWpiWWdzbjhwT1RMRXZpRGZDSmdIcmZtQmhNRWgy?=
 =?utf-8?B?RGVsc1lObDRvdEV0WWVRVTNxVDc0ay84UTJrbnJSdjF4a0VJMzhBdFNzdDAr?=
 =?utf-8?B?NENaV2g0T2RrUXFvTnRSdjRrSWtCYmpyT0pha29nUXhEZUlVUnNaR3JuZTdD?=
 =?utf-8?B?QVRBWDRKMnl1VmpOVmhCZzdlM0hYand3WGVKWGJGWTdjbjNLUjlRT3lnSHRh?=
 =?utf-8?B?R2dCckNQUHg3YUtYcWQrRzBJKzhzUTNKL0Z6eFZ0ejA2LzFVUm5lK2NDZmxj?=
 =?utf-8?B?QkwxZFJCaHI3bDN0N1V2WkhIeHg3SEJLTVdXY0tJOThIN3ZWcElyL1JNTmFp?=
 =?utf-8?B?czl5Ujd0VXZCa1pUSmlqT1A0QW9VenZCaFlRQmwzTWhuQndNL2Nvc1kzM3VI?=
 =?utf-8?B?bVo2VlUvbGViVHRtS25lVTJZdjNEKzFXYy9OTEJqdmprVFRnSnJPaWhPdG9z?=
 =?utf-8?B?b0lVSGpmSk1nSUdWRXJNV2RTY3IxZHhZL2oxRUdsRm9HS0ZlNDl4NWd5SURq?=
 =?utf-8?B?dkNtV1lobFRJQ2Z1R1NnYlZTSzlEb2NEM2FwWExOWFVJZVdzM0xqN0JqRllx?=
 =?utf-8?B?K0RWZWgyL3BRaVJSTFdNeTlpYURUZkZBcW5BblE2eHFSM1RJRWM3VlhVZGNP?=
 =?utf-8?B?YVpJd1FXRVpncHVBenEyOTR5N1VJYk9wSFFWUW9QSHdHYzcyd0RBRU5BdEg2?=
 =?utf-8?B?ZHZPcUtSbkxmbGpveVhmWFc1UzFDL0pBbjBqVlhwTitSeFJYeWNZbG9qNFND?=
 =?utf-8?B?b2pqK1hjRlFDOEROM3Z0UkFsYmcxcVVJSFlEcS8wcDduWVB2SmU0ZEYySFJo?=
 =?utf-8?B?S05OUWhpSks3UlZ0MGV1ZmF3RFRtR3lPN0VMdkRVUWFJeUpDU1hZN1BiMHl6?=
 =?utf-8?B?QXhjM00vZUhRRGJMVUV0aExJS1FGY2NzOWFTK2d3b3BQdXlISWhTdjdTd1pT?=
 =?utf-8?B?SE54REFtYWR2Vzl1N3VsYkhsSENZekdCQUFoM0hGUGlibW1hNDZsazB4K3U5?=
 =?utf-8?B?M2ZSVzNrQXliOTZ6V0N4UFNSWDFRV3lLbG5MZUtDOTlmWExIVmZneXFFcndx?=
 =?utf-8?B?d3AwT1lGd3BTZWh2bXcyRGlsMTNwYjM1TG94NDlxVjJuNHNTc1luRUFhWHNR?=
 =?utf-8?B?emdJaCtGVlFXZjYzYkduVE9aUGJJS1hKKytqSThLOFJQNEtBZjJTRWR2RmFi?=
 =?utf-8?B?amw2MnFoU2U4Yzl2L3FrZWNNUjRrRThVUVlsU3l5dVhVTFpQUmdRbHNyTjVJ?=
 =?utf-8?B?VjNuN0x3ZTFGQ0R6U25IV2ZuZGZHWXh4MTNsc0xZdUR4T2dsQTFRdUtsY21u?=
 =?utf-8?B?bERoZ0prdTBCcXg4anN3clNaZkF0RmtlUW1FdG1zT3N2ZTY2MXdHYUpVZ0lQ?=
 =?utf-8?B?RVNCcW9GeGE1d0JXSldyUzljYzlGUi85bnJiVlkwdXd5cXRJVGpWN1Q2eGUv?=
 =?utf-8?B?RmZOakVsdjVUcEdZZkVKVFN4MXljOW13YVlNSzU0RmNlZVFKVXkwMkV5czNB?=
 =?utf-8?B?bWwyOTlTTWsvTlRoYnQxdGVuU0Y4YkJpbWZoU3FDQWFmRjEzc2FYNER1dDAw?=
 =?utf-8?B?d0V6Qlc5dHZZaTByOHZIVFBrZnVPS2VSZmUzQVg1dis3UjJLU0hnTUZYTXpR?=
 =?utf-8?Q?XnDlVOg5gbP+YPVHJBAZ+tM=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: f3ceefae-4086-4e86-e262-08d9e4f48524
X-MS-Exchange-CrossTenant-AuthSource: BY5PR12MB4130.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Jan 2022 20:01:50.8919
 (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: YrgsQcYK7Xqx+jxn/u2LzUJSxhWK8jlq06ZD+Sh0BZ5LB8w4ArOtOzcdElJNm2sRlYHM1Ek6k6ddLlKKcZz6AQ==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR12MB2760
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 1/31/22 04:05, Jan Kara wrote:
> On Sun 30-01-22 21:17:52, John Hubbard wrote:
>> Unraveling the rat's nest set of APIs in mm/gup.c a bit more.
>> get_user_pages_locked() was not helping at all, so remove it.
>>
>> Also, lookup_node() has only a single caller, but it is still worth
>> having a clearer locking policy there. Changing it so that the caller
>> both takes and releases the mmap_lock, thus leaving lookup_node() with
>> the sole job of translating a virtual address into a numa node ID.
>>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> 
> Well, the point of _locked() GUP variants is that we can unlock mmap_sem
> when reading a page from the disk during a page fault (hidden behind
> VM_FAULT_RETRY). So as such _locked() variants are about reducing mmap_sem
> latency rather than code readability.  In this particular case, I don't
> think using _locked() variant in lookup_node() is very beneficial
> (generally I would not expect to take a fault there) but at least a
> justification in the commit message should be different :).
> 
> 								Honza

I'll rewrite this commit description to cover this point properly.
Jason also suggested using gup-fast, which I like.
  thanks,
-- 
John Hubbard
NVIDIA