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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.1 Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk Content-Language: en-US To: Peter Xu , , CC: Jann Horn , Andrea Arcangeli , James Houghton , Rik van Riel , Miaohe Lin , Nadav Amit , "Mike Kravetz" , David Hildenbrand , Andrew Morton , Muchun Song References: <20221207203034.650899-1-peterx@redhat.com> <20221207203158.651092-1-peterx@redhat.com> From: John Hubbard In-Reply-To: <20221207203158.651092-1-peterx@redhat.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.126.231.37] X-ClientProxiedBy: rnnvmail203.nvidia.com (10.129.68.9) To rnnvmail201.nvidia.com (10.129.68.8) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL02EPF00010206:EE_|BL1PR12MB5031:EE_ X-MS-Office365-Filtering-Correlation-Id: eb71f8d4-94c0-4263-3106-08dad8b171fb X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Dec 2022 00:16:25.7257 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: eb71f8d4-94c0-4263-3106-08dad8b171fb X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BL02EPF00010206.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL1PR12MB5031 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/7/22 12:31, Peter Xu wrote: > Taking vma lock here is not needed for now because all potential hugetlb > walkers here should have i_mmap_rwsem held. Document the fact. > > Reviewed-by: Mike Kravetz > Signed-off-by: Peter Xu > --- > mm/page_vma_mapped.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index e97b2e23bd28..2e59a0419d22 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -168,8 +168,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > /* The only possible mapping was handled on last iteration */ > if (pvmw->pte) > return not_found(pvmw); > - > - /* when pud is not present, pte will be NULL */ > + /* > + * NOTE: we don't need explicit lock here to walk the > + * hugetlb pgtable because either (1) potential callers of > + * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the > + * caller will not walk a hugetlb vma (e.g. ksm or uprobe). > + * When one day this rule breaks, one will get a warning > + * in hugetlb_walk(), and then we'll figure out what to do. > + */ Confused. Is this documentation actually intended to refer to hugetlb_walk() itself, or just this call site? If the former, then let's move it over to be right before hugetlb_walk(). > pvmw->pte = hugetlb_walk(vma, pvmw->address, size); > if (!pvmw->pte) > return false; thanks, -- John Hubbard NVIDIA