linux-kernel - Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL

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Message-ID: <CAKMK7uHvk+ti00mCCF2006U003w1dofFg9nSfmZ4bS2Z2pEDNQ@mail.gmail.com>
Date:   Wed, 13 Nov 2019 09:22:36 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
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Subject: Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM

On Tue, Nov 12, 2019 at 10:10 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 11/12/19 12:38 PM, Jason Gunthorpe wrote:
> > On Mon, Nov 11, 2019 at 04:06:37PM -0800, John Hubbard wrote:
> >> Hi,
> >>
> >> The cover letter is long, so the more important stuff is first:
> >>
> >> * Jason, if you or someone could look at the the VFIO cleanup (patch 8)
> >>   and conversion to FOLL_PIN (patch 18), to make sure it's use of
> >>   remote and longterm gup matches what we discussed during the review
> >>   of v2, I'd appreciate it.
> >>
> >> * Also for Jason and IB: as noted below, in patch 11, I am (too?) boldly
> >>   converting from put_user_pages() to release_pages().
> >
> > Why are we doing this? I think things got confused here someplace, as
>
>
> Because:
>
> a) These need put_page() calls,  and
>
> b) there is no put_pages() call, but there is a release_pages() call that
> is, arguably, what put_pages() would be.
>
>
> > the comment still says:
> >
> > /**
> >  * put_user_page() - release a gup-pinned page
> >  * @page:            pointer to page to be released
> >  *
> >  * Pages that were pinned via get_user_pages*() must be released via
> >  * either put_user_page(), or one of the put_user_pages*() routines
> >  * below.
>
>
> Ohhh, I missed those comments. They need to all be changed over to
> say "pages that were pinned via pin_user_pages*() or
> pin_longterm_pages*() must be released via put_user_page*()."
>
> The get_user_pages*() pages must still be released via put_page.
>
> The churn is due to a fairly significant change in strategy, whis
> is: instead of changing all get_user_pages*() sites to call
> put_user_page(), change selected sites to call pin_user_pages*() or
> pin_longterm_pages*(), plus put_user_page().

Can't we call this unpin_user_page then, for some symmetry? Or is that
even more churn?

Looking from afar the naming here seems really confusing.
-Daniel

> That allows incrementally converting the kernel over to using the
> new pin APIs, without taking on the huge risk of a big one-shot
> conversion.
>
> So, I've ended up with one place that actually needs to get reverted
> back to get_user_pages(), and that's the IB ODP code.
>
> >
> > I feel like if put_user_pages() is not the correct way to undo
> > get_user_pages() then it needs to be deleted.
> >
>
> Yes, you're right. I'll fix the put_user_page comments() as described.
>
>
> thanks,
>
> John Hubbard
> NVIDIA



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch