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

[David Hildenbrand]

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