linux-kernel - Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned

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Date:   Thu, 5 Jul 2018 14:17:19 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Jan Kara <jack@...e.cz>
cc:     John Hubbard <jhubbard@...dia.com>, john.hubbard@...il.com,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] mm: track gup pages with page->dma_pinned_*
 fields

On Wed, 4 Jul 2018, Jan Kara wrote:

> > So this seems unsolvable without having the caller specify that it knows the
> > page type, and that it is therefore safe to decrement page->dma_pinned_count.
> > I was hoping I'd found a way, but clearly I haven't. :)
>
> Well, I think the misconception is that "pinned" is a fundamental property
> of a page. It is not. "pinned" is a property of a page reference (i.e., a
> kind of reference that can be used for DMA access) and page gets into
> "pinned" state if it has any reference of "pinned" type. And when you
> realize this, it is obvious that you just have to have a special api for
> getting and dropping references of this "pinned" type. For getting we
> already have get_user_pages(), for putting we have to create the api...

Maybe we can do something by creating a special "pinned" bit in the pte?
If it is a RDMA reference then set that pinned bit there.

Thus any of the references could cause a pin. Since the page struct does
not contain that information we therefore have to scan through the ptes to
figure out if a page is pinned?

If so then we would not need a special function for dropping the
reference.

References to a page can also be created from devices mmu. Maybe we could
at some point start to manage them in a similar way to the page tables of
the processor? The mmu notifiers are a bit awkward if we need closer mm
integration.