xen/gntdev: Use VM_MIXEDMAP instead of VM_IO to avoid NUMA balancing

Commit 9c17d96500f7 ("xen/gntdev: Grant maps should not be subject to
NUMA balancing") set VM_IO flag to prevent grant maps from being
subjected to NUMA balancing.

It was discovered recently that this flag causes get_user_pages() to
always fail with -EFAULT.

check_vma_flags
__get_user_pages
__get_user_pages_locked
__get_user_pages_unlocked
get_user_pages_fast
iov_iter_get_pages
dio_refill_pages
do_direct_IO
do_blockdev_direct_IO
do_blockdev_direct_IO
ext4_direct_IO_read
generic_file_read_iter
aio_run_iocb

(which can happen if guest's vdisk has direct-io-safe option).

To avoid this let's use VM_MIXEDMAP flag instead --- it prevents
NUMA balancing just as VM_IO does and has no effect on
check_vma_flags().

Cc: stable@vger.kernel.org

Reported-by: Olaf Hering <olaf@aepfle.de>
Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Hugh Dickins <hughd@google.com>
Tested-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/gntdev.c

@@ -1007,7 +1007,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 
 	vma->vm_ops = &gntdev_vmops;
 
-	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_IO;
+	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP | VM_MIXEDMAP;
 
 	if (use_ptemod)
 		vma->vm_flags |= VM_DONTCOPY;