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

From: Juergen Gross
Date: Mon Nov 28 2016 - 08:17:11 EST


On 21/11/16 15:56, 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>

Applied to xen/tip.git for-linus-4.10


Juergen

> ---
> 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;
>