Re: [PATCH v4] xen/gntdev: Use VM_MIXEDMAP instead of VM_IO to avoid NUMA balancing

[Hugh Dickins]

On Mon, 21 Nov 2016, Boris Ostrovsky wrote:

> 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().
> 
> Reported-by: Olaf Hering <olaf@xxxxxxxxx>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

But nicer with the addition of a code comment saying
	/* VM_MIXEDMAP to forbid NUMA balancing but permit get_user_pages() */

> ---
>  drivers/xen/gntdev.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index bb95212..2ef2b61 100644
> --- a/drivers/xen/gntdev.c
> +++ b/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;
> -- 
> 1.7.1