linux-kernel - Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL

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Message-ID: <CAPcyv4hdYZ__3+KJHh+0uX--f-U=pLiZfdO0JDhyBE-nZ=i4FQ@mail.gmail.com>
Date:   Tue, 12 Nov 2019 17:35:46 -0800
From:   Dan Williams <dan.j.williams@...el.com>
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>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

On Tue, Nov 12, 2019 at 5:08 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 11/12/19 4:58 PM, Dan Williams wrote:
> ...
> >>> It's not redundant relative to upstream which does not do anything the
> >>> FOLL_LONGTERM in the gup-slow path... but I have not looked at patches
> >>> 1-7 to see if something there made it redundant.
> >>
> >> Oh, the hunk John had below for get_user_pages_remote() also needs to
> >> call __gup_longterm_locked() when FOLL_LONGTERM is specified, then
> >> that calls check_dax_vmas() which duplicates the vma_is_fsdax() check
> >> above.
> >
> > Oh true, good eye. It is redundant if it does additionally call
> > __gup_longterm_locked(), and it needs to do that otherwises it undoes
> > the CMA migration magic that Aneesh added.
> >
>
> OK. So just to be clear, I'll be removing this from the patch:
>
>         /*
>          * The lifetime of a vaddr_get_pfn() page pin is
>          * userspace-controlled. In the fs-dax case this could
>          * lead to indefinite stalls in filesystem operations.
>          * Disallow attempts to pin fs-dax pages via this
>          * interface.
>          */
>         if (ret > 0 && vma_is_fsdax(vmas[0])) {
>                 ret = -EOPNOTSUPP;
>                 put_page(page[0]);
>         }
>
> (and the declaration of "vmas", as well).

...and add a call to __gup_longterm_locked internal to
get_user_pages_remote(), right?