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

[Juergen Gross]

On 22/11/16 04:59, Hugh Dickins wrote:
> 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() */

I can fix that up when committing.


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;
>> -- 
>> 1.7.1
>