linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Mon, 18 Jun 2018 10:56:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
        john.hubbard@...il.com, Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>, Jan Kara <jack@...e.cz>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

On Mon, Jun 18, 2018 at 10:50 AM, John Hubbard <jhubbard@...dia.com> wrote:
> On 06/18/2018 01:12 AM, Christoph Hellwig wrote:
>> On Sun, Jun 17, 2018 at 01:28:18PM -0700, John Hubbard wrote:
>>> Yes. However, my thinking was: get_user_pages() can become a way to indicate that
>>> these pages are going to be treated specially. In particular, the caller
>>> does not really want or need to support certain file operations, while the
>>> page is flagged this way.
>>>
>>> If necessary, we could add a new API call.
>>
>> That API call is called get_user_pages_longterm.
>
> OK...I had the impression that this was just semi-temporary API for dax, but
> given that it's an exported symbol, I guess it really is here to stay.

The plan is to go back and provide api changes that bypass
get_user_page_longterm() for RDMA. However, for VFIO and others, it's
not clear what we could do. In the VFIO case the guest would need to
be prepared handle the revocation.