Received-SPF: pass (google.com: domain of linux-kernel+bounces-150940-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <b05cdac2-84f2-4727-af6c-3b666e6add14@intel.com>
Date: Fri, 19 Apr 2024 10:29:44 +0800
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using
 the mapcount of large folios
To: David Hildenbrand <david@redhat.com>, <linux-kernel@vger.kernel.org>
CC: <linux-mm@kvack.org>, <linux-doc@vger.kernel.org>,
	<cgroups@vger.kernel.org>, <linux-sh@vger.kernel.org>,
	<linux-trace-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>, Andrew
 Morton <akpm@linux-foundation.org>, "Matthew Wilcox (Oracle)"
	<willy@infradead.org>, Peter Xu <peterx@redhat.com>, Ryan Roberts
	<ryan.roberts@arm.com>, Yang Shi <shy828301@gmail.com>, Zi Yan
	<ziy@nvidia.com>, Jonathan Corbet <corbet@lwn.net>, Hugh Dickins
	<hughd@google.com>, Yoshinori Sato <ysato@users.sourceforge.jp>, Rich Felker
	<dalias@libc.org>, John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>, "Muchun
 Song" <muchun.song@linux.dev>, Miaohe Lin <linmiaohe@huawei.com>, "Naoya
 Horiguchi" <naoya.horiguchi@nec.com>, Richard Chang <richardycc@google.com>
References: <20240409192301.907377-1-david@redhat.com>
 <20240409192301.907377-6-david@redhat.com>
Content-Language: en-US
From: "Yin, Fengwei" <fengwei.yin@intel.com>
In-Reply-To: <20240409192301.907377-6-david@redhat.com>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?WDF3OVJzOURXNHpvcHpiTmVhamxLNnpWU1lMTjNSdXg5cXpPcWlMaktpWk03?=
 =?utf-8?B?VUhYUllkNWw5UVVyd3d6UnNmMjBpNFk0WllWZ2tWU1YvTUkvejFwU0N0ODAy?=
 =?utf-8?B?VFBKblFDdUlIclh0V0I5QllwaCsxMm9ySUFKcDVpZjJEMXMwS0xYdVQva0xh?=
 =?utf-8?B?SVVtMjJPZlMxMmtVYnhoYjJOZU1JSm5kQzE0WHhJL2VicjJ2Q3gxUzRBbklF?=
 =?utf-8?B?dDRaMG1WTmtSMzFJL3dZM2dQMGNrVW9YTVRucXJxakJNYjI3MXo2ZnFQRjYy?=
 =?utf-8?B?YldlaEp0OWRFdHVURHRWOUNia1phWDNkZm9nUmVJREFteTFqdmxETFZiUkNJ?=
 =?utf-8?B?djNNSEFLMzRuTkVNMHZjTVFiMWRnZm1Zb1lKciszZm5CQWtlOUI4MmZGSE9s?=
 =?utf-8?B?Vjh4czFFanJBTzdqeklDOXhqU1NsaE5sSFE0OWcvQm8yVG5zZEJnZUk1M3M4?=
 =?utf-8?B?ZHhxZml0dDEwelA3byt3eXArc0phRU11NFNJcldRbS82ZmdYSUk5SlZTNmlH?=
 =?utf-8?B?cUZhOUZ4dXV0cWlzVkNhb1g2R2hxa2doalBDR3pGWEN5NUNLZnAvUFgxMW1T?=
 =?utf-8?B?a1JqWFJCSWMzdXNDVVdIaEdqTngwdnVZY0lZdDgyZGNWUzBMdjRWRis4VEhU?=
 =?utf-8?B?bHFGT21DdlEwMFhVYzFVL0VxajNwR1JLRzdFa2hYcC9sL0ZKTVBnV2tlZ1Jn?=
 =?utf-8?B?eGVQQUZJUUwrQjZYczVZQlBML2JLL3NvRlM5cU44MTBVbkhRYVZtbWl0VFk3?=
 =?utf-8?B?MFQ2ZEVsNklmbWlKTVNCTnI4TGZaYjJsQ3FtVVlrbkJ4b1YwNGJrdnN6S29N?=
 =?utf-8?B?VU94cGRvcWx6ZVQ2NjdzY1MrQ1hLMjdKaGhYamZ5RWRtYzN0TlpjMGxxNCtN?=
 =?utf-8?B?SWh6VFZLZ3RFYmdkMTVBRkJBc0R1SkpISzA5Nk94Q3BRQ2w0azY4UnNHdmMw?=
 =?utf-8?B?REEvcFNoQUZRTFErdW05OXUydGdVY2JRNytoQ0NwT2ZiZU5nYlllQ0FoMCtt?=
 =?utf-8?B?M1JYR2lFWDQ2bG1aS2ZPZEtFdFRBRnJ4WUJGdmE5L3NhV3FEekZaNVB3MWsr?=
 =?utf-8?B?dGxoN2lzS2c5aDh5RUlneFphTVgva2c0TFMzZmtHK3c5NjRJbnNIbXg2UnIy?=
 =?utf-8?B?cm81Z2pPZ2hkRWFFWXZ2RSt2ano5d0FVZVNLY1dJWDYybHAvU1ArWWZTMmhZ?=
 =?utf-8?B?bkkyU0FGNnZhVmV0alJWaHcrbDZKWlJZdmdFRld3cGdTOWtPMEtsSDhUYWps?=
 =?utf-8?B?TWdwOGp2TmQ2U0hnOGhvQUdzQWZLRnNUMC9xT1g5Nis3UGRZeFUzWjRWK2tz?=
 =?utf-8?B?R1NXMmRHRFFDc0dRQlBzM2dXYmpMWnBiOEhTdU44NWJRNmxxWnQvMFYvOTYw?=
 =?utf-8?B?SlYwb201Ky9pUGdkMkxoR2R2UEIxU1ZqSG55K0poR2ZOUE1VR3N6OS9PYVNz?=
 =?utf-8?B?M2N2K3FpTkFuLzRtVnI5WGtiZmE2cUhocWdWeVc1dWl1d0oxN2JXb2w4NDE3?=
 =?utf-8?B?cDFDNTZzbFR5Zk9HWGNJL1BHVTJFV0cvdTM1SDVKTGNqSzNXYzRlT0FsaVIv?=
 =?utf-8?B?MEVOUzB6YTUvaER0UFJLS3hRdWdkZmZrZExRanA0eDFsSnRITzd1MzdmM3d1?=
 =?utf-8?B?RFNpOHBwS0lKK3JzRWM0cGVTWFBCT1E3VEZFSkhuM3gvSVk2N0NCN2RnemFo?=
 =?utf-8?B?ZXlJSnFnRDlxempYMk5nVUdVQndYNEJjOVJ6T0NXT0M4aEs2eE82c1FsK0ps?=
 =?utf-8?B?RVUzSjB0M2xUanUwVkQ5YXJKM0lzOU44TzBYT2VUeFJUamxmZmpwcFNDcXNX?=
 =?utf-8?B?YTFMUGNUbEp4bFZydDZ0SzBCdGRGVGp0Vk1uZyt4NlA3b0ltYXF0clc1cWc3?=
 =?utf-8?B?SDYzRW9sZ3RLS25kRTBVV05sc3NaZDJ2VFI2K3VLUHdxWXlRNEd1Zi8zeU5Z?=
 =?utf-8?B?MTZSMHpHRCtpd1dlNGlabTdzbk5pVXVOVDNPYkswTFZKZ2lZa1c4L2RqaWVj?=
 =?utf-8?B?UGNpSlFBOUNvUjZBTSsvUXFLcFRISTVyNGFRRlRIZGk5aVJkcUVvQnlLL0JQ?=
 =?utf-8?B?QjdNeVVlbnIvNHBnQ1NoWlFzcXRlcm1NYUc2T3Zqam9Wd0UvSURFeUZJbWtz?=
 =?utf-8?B?aDl3RHk2SitxQ0JEZG1ySlh6QkZFTGF4NzVuSnpLdFpDaFVsaDNHNkJkQU1P?=
 =?utf-8?B?SXc9PQ==?=
X-MS-Exchange-CrossTenant-Network-Message-Id: b058591c-ae7d-4946-34a4-08dc60189a9c
X-MS-Exchange-CrossTenant-AuthSource: SJ0PR11MB4831.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Apr 2024 02:29:57.1608
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: /joS9ywHRkmfJFvVwa0NveN8yyVJDvkRtA6DWn4aaTy1+RFYrG0JMBlWNaxKK4uncT/M2xZkd8IbEyyMQ2e7tQ==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR11MB8752
X-OriginatorOrg: intel.com


On 4/10/2024 3:22 AM, David Hildenbrand wrote:
> @@ -2200,7 +2200,22 @@ static inline size_t folio_size(struct folio *folio)
>    */
>   static inline bool folio_likely_mapped_shared(struct folio *folio)
>   {
> -	return page_mapcount(folio_page(folio, 0)) > 1;
> +	int mapcount = folio_mapcount(folio);
> +
> +	/* Only partially-mappable folios require more care. */
> +	if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
> +		return mapcount > 1;
My understanding is that mapcount > folio_nr_pages(folio) can cover
order 0 folio. And also folio_entire_mapcount() can cover hugetlb (I am
not 100% sure for this one).  I am wondering whether we can drop above
two lines? Thanks.


Regards
Yin, Fengwei

> +
> +	/* A single mapping implies "mapped exclusively". */
> +	if (mapcount <= 1)
> +		return false;
> +
> +	/* If any page is mapped more than once we treat it "mapped shared". */
> +	if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
> +		return true;
> +
> +	/* Let's guess based on the first subpage. */
> +	return atomic_read(&folio->_mapcount) > 0;
>   }