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Tsirkin" , Jason Wang , =?utf-8?Q?C=C3=A9dric?= Le Goater , dimitri.sivanich@hpe.com Subject: Re: [PATCH v4 0/7] introduce vm_flags modifier functions Message-ID: References: <20230126193752.297968-1-surenb@google.com> <20230314141144.6a0892e6.alex.williamson@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230314141144.6a0892e6.alex.williamson@redhat.com> X-ClientProxiedBy: MN2PR19CA0059.namprd19.prod.outlook.com (2603:10b6:208:19b::36) To LV2PR12MB5869.namprd12.prod.outlook.com (2603:10b6:408:176::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV2PR12MB5869:EE_|SJ2PR12MB7962:EE_ X-MS-Office365-Filtering-Correlation-Id: 1d463ae1-340e-4fb2-6282-08db2adc0df1 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:LV2PR12MB5869.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230025)(4636009)(346002)(39860400002)(136003)(366004)(376002)(396003)(451199018)(2616005)(478600001)(83380400001)(38100700002)(6486002)(316002)(6512007)(54906003)(7406005)(186003)(7366002)(6506007)(7416002)(8936002)(86362001)(4326008)(66946007)(6916009)(26005)(66476007)(2906002)(5660300002)(41300700001)(66556008)(8676002)(36756003);DIR:OUT;SFP:1101; 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It is now a preprequisite for the next version of per-VMA > > lock patchset, which reuses vm_flags modifier functions to lock the VMA when > > vm_flags are being updated. > > > > VMA vm_flags modifications are usually done under exclusive mmap_lock > > protection because this attrubute affects other decisions like VMA merging > > or splitting and races should be prevented. Introduce vm_flags modifier > > functions to enforce correct locking. > > > > The patchset applies cleanly over mm-unstable branch of mm tree. > > With this series, vfio-pci developed a bunch of warnings around not > holding the mmap_lock write semaphore while calling > io_remap_pfn_range() from our fault handler, vfio_pci_mmap_fault(). > > I suspect vdpa has the same issue for their use of remap_pfn_range() > from their fault handler, JasonW, MST, FYI. Yeah, IMHO this whole approach has always been a bit sketchy, it was never intended that remap_pfn would be called from a fault handler, you are supposed to use a vmf_insert_pfn() type API from fault handlers. > The reason for using remap_pfn_range() on fault in vfio-pci is that > we're mapping device MMIO to userspace, where that MMIO can be disabled > and we'd rather zap the mapping when that occurs so that we can sigbus > the user rather than allow the user to trigger potentially fatal bus > errors on the host. > Peter Xu has suggested offline that a non-lazy approach to reinsert the > mappings might be more inline with mm expectations relative to touching > vm_flags during fault. Yes, I feel the same - along with completing the address_space conversion you had started. IIRC that was part of the reason we needed this design in vfio. Jason