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

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Date:   Fri, 18 Nov 2016 14:27:12 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        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 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.

Hugh