Re: [PATCH v3 (re-send)] xen/gntdev: Use mempolicy instead of VM_IO flag to avoid NUMA balancing

From: Boris Ostrovsky
Date: Fri Nov 18 2016 - 17:48:27 EST

On 11/18/2016 05:27 PM, Hugh Dickins wrote:
> On Fri, 18 Nov 2016, Boris Ostrovsky wrote:
>> On 11/18/2016 04:51 PM, Hugh Dickins wrote:
>>> Hmm, sorry, but this seems overcomplicated to me: ingenious, but an
>>> unusual use of the ->get_policy method, which is a little worrying,
>>> since it has only been used for shmem (+ shm and kernfs) until now.
>>> Maybe I'm wrong, but wouldn't substituting VM_MIXEDMAP for VM_IO
>>> solve the problem more simply?
>> It would indeed. I didn't want to use it because it has specific meaning
>> ("Can contain "struct page" and pure PFN pages") and that didn't seem
>> like the right flag to describe this vma.
> It is okay if it contains 0 pure PFN pages; and no worse than VM_IO was.
> A comment on why VM_MIXEDMAP is being used there would certainly be good.
> But I do find its use preferable to enlisting an unusual ->get_policy.

OK, I'll set VM_MIXEDMAP then.

I am still curious though why you feel get_policy is not appropriate
here (beside the fact that so far it had limited use). It is essentially
trying to say that the only policy to be consulted (in vma_policy_mof())
is of the vma itself and not of the task.