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

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Message-ID: <20191112234250.GA19615@ziepe.ca>
Date:   Tue, 12 Nov 2019 19:42:50 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        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 02:45:51PM -0800, Dan Williams wrote:
> On Tue, Nov 12, 2019 at 2:43 PM John Hubbard <jhubbard@...dia.com> wrote:
> >
> > On 11/12/19 12:43 PM, Jason Gunthorpe wrote:
> > ...
> > >> -            }
> > >> +    ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
> > >> +                                page, vmas, NULL);
> > >> +    /*
> > >> +     * 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]);
> > >>      }
> > >
> > > AFAIK this chunk is redundant now as it is some hack to emulate
> > > FOLL_LONGTERM? So vmas can be deleted too.
> >
> > Let me first make sure I understand what Dan has in mind for the vma
> > checking, in the other thread...
> 
> 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.

Certainly no caller of FOLL_LONGTERM should have to do dax specific
VMA checking.

Jason