linux-kernel - Re: [PATCH v3 (re-send)] xen/gntdev: Use mempolicy instead of VM

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Message-ID: <05c24d23-0298-5b58-d0e8-095ba64cdf9b@oracle.com>
Date:   Fri, 18 Nov 2016 17:49:41 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>, david.vrabel@...rix.com,
        jgross@...e.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, olaf@...fle.de
Subject: Re: [PATCH v3 (re-send)] xen/gntdev: Use mempolicy instead of VM_IO
 flag to avoid NUMA balancing

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.

-boris