Subject: Re: [PATCH v3 (re-send)] xen/gntdev: Use mempolicy instead of VM_IO
 flag to avoid NUMA balancing
To: Hugh Dickins <hughd@google.com>
References: <1479413404-27332-1-git-send-email-boris.ostrovsky@oracle.com>
 <alpine.LSU.2.11.1611181335560.9605@eggly.anvils>
 <2bf041f3-8918-3c6f-8afb-c9edcc03dcd9@oracle.com>
 <alpine.LSU.2.11.1611181421470.10145@eggly.anvils>
Cc: Mel Gorman <mgorman@techsingularity.net>, david.vrabel@citrix.com,
        jgross@suse.com, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, olaf@aepfle.de
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <05c24d23-0298-5b58-d0e8-095ba64cdf9b@oracle.com>
Date: Fri, 18 Nov 2016 17:49:41 -0500
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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