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

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Date:   Tue, 3 Jul 2018 17:08:12 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     John Hubbard <jhubbard@...dia.com>
cc:     Jan Kara <jack@...e.cz>, 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 Mon, 2 Jul 2018, John Hubbard wrote:

> > If you establish a reference to a page then increase the page count. If
> > the reference is a dma pin action also then increase the pinned count.
> >
> > That way you know how many of the references to the page are dma
> > pins and you can correctly manage the state of the page if the dma pins go
> > away.
> >
>
> I think this sounds like what this patch already does, right? See:
> __put_page_for_pinned_dma(), __get_page_for_pinned_dma(), and
> pin_page_for_dma(). The locking seems correct to me, but I suspect it's
> too heavyweight for such a hot path. But without adding a new put_user_page()
> call, that was the best I could come up with.

When I saw the patch it looked like you were avoiding to increment the
page->count field.

> What I'm hearing now from Jan and Michal is that the desired end result is
> a separate API call, put_user_pages(), so that we can explicitly manage
> these pinned pages.

Certainly a good approach.