Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

David Hildenbrand
2022-11-29T11:40:11Z

On 29.11.22 12:21, Kirill A. Shutemov wrote:
> On Mon, Nov 28, 2022 at 06:06:32PM -0600, Michael Roth wrote:
>> On Tue, Oct 25, 2022 at 11:13:37PM +0800, Chao Peng wrote:
>>> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>>>
>>
>> <snip>
>>
>>> +static struct file *restrictedmem_file_create(struct file *memfd)
>>> +{
>>> + struct restrictedmem_data *data;
>>> + struct address_space *mapping;
>>> + struct inode *inode;
>>> + struct file *file;
>>> +
>>> + data = kzalloc(sizeof(*data), GFP_KERNEL);
>>> + if (!data)
>>> + return ERR_PTR(-ENOMEM);
>>> +
>>> + data->memfd = memfd;
>>> + mutex_init(&data->lock);
>>> + INIT_LIST_HEAD(&data->notifiers);
>>> +
>>> + inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
>>> + if (IS_ERR(inode)) {
>>> + kfree(data);
>>> + return ERR_CAST(inode);
>>> + }
>>> +
>>> + inode->i_mode |= S_IFREG;
>>> + inode->i_op = &restrictedmem_iops;
>>> + inode->i_mapping->private_data = data;
>>> +
>>> + file = alloc_file_pseudo(inode, restrictedmem_mnt,
>>> + "restrictedmem", O_RDWR,
>>> + &restrictedmem_fops);
>>> + if (IS_ERR(file)) {
>>> + iput(inode);
>>> + kfree(data);
>>> + return ERR_CAST(file);
>>> + }
>>> +
>>> + file->f_flags |= O_LARGEFILE;
>>> +
>>> + mapping = memfd->f_mapping;
>>> + mapping_set_unevictable(mapping);
>>> + mapping_set_gfp_mask(mapping,
>>> + mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);
>>
>> Is this supposed to prevent migration of pages being used for
>> restrictedmem/shmem backend?
>
> Yes, my bad. I expected it to prevent migration, but it is not true.

Maybe add a comment that these pages are not movable and we don't want
to place them into movable pageblocks (including CMA and ZONE_MOVABLE).
That's the primary purpose of the GFP mask here.

--
Thanks,

David / dhildenb