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Tue, 4 Nov 2025 09:27:46 +0000 Date: Tue, 4 Nov 2025 17:25:44 +0800 From: Yan Zhao To: Ackerley Tng CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [RFC PATCH v1 11/37] KVM: guest_memfd: Add support for KVM_SET_MEMORY_ATTRIBUTES Message-ID: Reply-To: Yan Zhao References: <5a4dfc265a46959953e6c24730d22584972b1179.1760731772.git.ackerleytng@google.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <5a4dfc265a46959953e6c24730d22584972b1179.1760731772.git.ackerleytng@google.com> X-ClientProxiedBy: KU2P306CA0060.MYSP306.PROD.OUTLOOK.COM (2603:1096:d10:39::9) To DS7PR11MB5966.namprd11.prod.outlook.com (2603:10b6:8:71::6) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR11MB5966:EE_|SA1PR11MB6568:EE_ X-MS-Office365-Filtering-Correlation-Id: 7e3446ce-c399-4550-e293-08de1b846a28 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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The filemap_invalidate_lock() is held, so no faulting > can occur. Hence, from that point on, only transient refcounts can be > taken on the folios associated with that guest_memfd. > > Hence, it is safe to do the conversion from shared to private. > > After conversion is complete, refcounts may become elevated, but that > is fine since users of transient refcounts don't actually access > memory. > > For private to shared conversions, there are no refcount checks. any > transient refcounts are expected to drop their refcounts soon. The > conversion process will spin waiting for these transient refcounts to > go away. Where's the code to spin? > +/* > + * Preallocate memory for attributes to be stored on a maple tree, pointed to > + * by mas. Adjacent ranges with attributes identical to the new attributes > + * will be merged. Also sets mas's bounds up for storing attributes. > + * > + * This maintains the invariant that ranges with the same attributes will > + * always be merged. > + */ > +static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes, > + pgoff_t start, size_t nr_pages) > +{ > + pgoff_t end = start + nr_pages; > + pgoff_t last = end - 1; > + void *entry; > + > + /* Try extending range. entry is NULL on overflow/wrap-around. */ > + mas_set_range(mas, end, end); > + entry = mas_find(mas, end); > + if (entry && xa_to_value(entry) == attributes) > + last = mas->last; > + > + mas_set_range(mas, start - 1, start - 1); Check start == 0 ? > + entry = mas_find(mas, start - 1); > + if (entry && xa_to_value(entry) == attributes) > + start = mas->index; > + > + mas_set_range(mas, start, last); > + return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL); > +} ... > +static long kvm_gmem_set_attributes(struct file *file, void __user *argp) > +{ > + struct gmem_file *f = file->private_data; > + struct inode *inode = file_inode(file); > + struct kvm_memory_attributes2 attrs; > + pgoff_t err_index; > + size_t nr_pages; > + pgoff_t index; > + int r; > + > + if (copy_from_user(&attrs, argp, sizeof(attrs))) > + return -EFAULT; > + > + if (attrs.flags) > + return -EINVAL; > + if (attrs.attributes & ~kvm_supported_mem_attributes(f->kvm)) > + return -EINVAL; > + if (attrs.size == 0 || attrs.offset + attrs.size < attrs.offset) > + return -EINVAL; > + if (!PAGE_ALIGNED(attrs.offset) || !PAGE_ALIGNED(attrs.offset)) Should be if (!PAGE_ALIGNED(attrs.offset) || !PAGE_ALIGNED(attrs.size)) ? > + return -EINVAL; > + > + if (attrs.offset > inode->i_size || Should be if (attrs.offset >= inode->i_size || ? > + attrs.offset + attrs.size > inode->i_size) > + return -EINVAL; > + > + nr_pages = attrs.size >> PAGE_SHIFT; > + index = attrs.offset >> PAGE_SHIFT; > + r = __kvm_gmem_set_attributes(inode, index, nr_pages, attrs.attributes, > + &err_index); > + if (r) { > + attrs.error_offset = err_index << PAGE_SHIFT; > + > + if (copy_to_user(argp, &attrs, sizeof(attrs))) > + return -EFAULT; > + } > + > + return r; > +}