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linux-kernel@vger.kernel.org, David Hildenbrand Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() Message-ID: <20230815211226.GA4150@monkey> References: <5BBFF5D3-3416-4C0E-9FDD-655661657D67@nvidia.com> <20230809205316.GA3537@monkey> <20230809224424.GB3537@monkey> <2da95492-079b-43b1-a950-d290984a21c0@huawei.com> <20230810162920.GA4734@monkey> <87wmxx7y9j.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87wmxx7y9j.fsf@yhuang6-desk2.ccr.corp.intel.com> X-ClientProxiedBy: MW2PR16CA0033.namprd16.prod.outlook.com (2603:10b6:907::46) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY5PR10MB4196:EE_|SJ0PR10MB5892:EE_ X-MS-Office365-Filtering-Correlation-Id: c4eedf4b-835a-43e6-b854-08db9dd45514 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.267,Aquarius:18.0.957,Hydra:6.0.601,FMLib:17.11.176.26 definitions=2023-08-15_19,2023-08-15_02,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 phishscore=0 mlxscore=0 bulkscore=0 mlxlogscore=999 suspectscore=0 spamscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2306200000 definitions=main-2308150189 X-Proofpoint-GUID: zvO0wanXDyaShlRbVlEB3UL9kewBgJQW X-Proofpoint-ORIG-GUID: zvO0wanXDyaShlRbVlEB3UL9kewBgJQW Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/15/23 11:58, Huang, Ying wrote: > Mike Kravetz writes: > > > On 08/10/23 09:49, Kefeng Wang wrote: > >> > >> > >> On 2023/8/10 6:44, Mike Kravetz wrote: > >> > On 08/09/23 13:53, Mike Kravetz wrote: > >> > > On 08/09/23 20:37, Kefeng Wang wrote: > >> > > > > > >> > > > > Cc Mike to help us clarify the expected behavior of hugetlb. > >> > > > > > >> > > > > Hi Mike, what is the expected behavior, if a user tries to use move_pages() > >> > > > > to migrate a non head page of a hugetlb page? > >> > > > > >> > > > Could you give some advise, thanks > >> > > > > >> > > > >> > > Sorry, I was away for a while. > >> > > > >> > > It seems unfortunate that move_pages says the passed user addresses > >> > > should be aligned to page boundaries. However, IIUC this is not checked > >> > > or enforced. Otherwise, passing a hugetlb page should return the same > >> > > error. > >> > > > >> > > One thought would be that hugetlb mappings should behave the same > >> > > non-hugetlb mappings. If passed the address of a hugetlb tail page, align > >> > > the address to a hugetlb boundary and migrate the page. This changes the > >> > > existing behavior. However, it would be hard to imagine anyone depending > >> > > on this. > >> > > > >> > > After taking a closer look at the add_page_for_migration(), it seems to > >> > > just ignore passed tail pages and do nothing for such passed addresses. > >> > > Correct? Or, am I missing something? Perhaps that is behavior we want/ > >> > > need to preserve? > >> > > >> > My mistake, status -EACCES is returned when passing a tail page of a > >> > hugetlb page. > >> > > >> > >> As mentioned in previous mail， before e66f17ff7177 ("mm/hugetlb: take > >> page table lock in follow_huge_pmd()") in v4.0, follow_page() will > >> return NULL on tail page for Huagetlb page, so move_pages() will return > >> -ENOENT errno, but after that commit, -EACCES is returned. > >> > >> Meanwhile, the behavior of THP/HUGETLB is different, the whole THP will be > >> migrated on a tail page, but HUGETLB will return -EACCES(after v4.0) > >> or -ENOENT(before v4.0) on tail page. > >> > >> > Back to the question of 'What is the expected behavior if a tail page is > >> > passed?'. I do not think we have defined an expected behavior. If > >> > anything is 'expected' I would say it is -EACCES as returned today. > >> > > >> > >> My question is, > >> > >> Should we keep seem behavior between HUGETLB and THP, or only change the > >> errno from -EACCES to -ENOENT/-EBUSY. > > > > Just to be clear. When you say "keep seem behavior between HUGETLB and THP", > > are you saying that you would like hugetlb to perform migration of the entire > > hugetlb page if a tail page is passed? > > > > IMO, this would be ideal as it would mean that hugetlb and THP behave the same > > when passed the address of a tail page. The fewer places where hugetlb > > behavior diverges, the better. However, this does change behavior. > > A separate patch will be needed for behavior change. > Correct. Since the goal of this series is to convert to folios, we should maintain the existing behavior and errno (-EACCES). In a subsequent patch, we can change behavior. That would be my suggestion. -- Mike Kravetz