linux-kernel - Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL

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Date:   Mon, 4 Nov 2019 17:15:25 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jerome Glisse <jglisse@...hat.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>,
        Dan Williams <dan.j.williams@...el.com>,
        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>,
        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>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN

On Mon, Nov 04, 2019 at 12:57:59PM -0800, John Hubbard wrote:
> On 11/4/19 12:37 PM, Jason Gunthorpe wrote:
> > On Mon, Nov 04, 2019 at 03:31:53PM -0500, Jerome Glisse wrote:
> >>> Note for Jason: the (a) or (b) items are talking about the vfio case, which is
> >>> one of the two call sites that now use pin_longterm_pages_remote(), and the
> >>> other one is infiniband:
> >>>
> >>> drivers/infiniband/core/umem_odp.c:646:         npages = pin_longterm_pages_remote(owning_process, owning_mm,
> >>> drivers/vfio/vfio_iommu_type1.c:353:            ret = pin_longterm_pages_remote(NULL, mm, vaddr, 1,
> >>
> >> vfio should be reverted until it can be properly implemented.
> >> The issue is that when you fix the implementation you might
> >> break vfio existing user and thus regress the kernel from user
> >> point of view. So i rather have the change to vfio reverted,
> >> i believe it was not well understood when it got upstream,
> >> between in my 5.4 tree it is still gup_remote not longterm.
> > 
> > It is clearly a bug, vfio must use LONGTERM, and does right above this
> > remote call:
> > 
> >         if (mm == current->mm) {
> >                 ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
> >                                      vmas);
> >         } else {
> >                 ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
> >                                             vmas, NULL);
> > 
> > 
> > I'm not even sure that it really makes any sense to build a 'if' like
> > that, surely just always call remote??
> > 
> 
> 
> Right, and I thought about this when converting, and realized that the above 
> code is working around the current gup.c limitations, which are "cannot support
> gup remote with FOLL_LONGTERM".

But AFAICT it doesn't have a problem, the protection test is just too
strict, and I guess the control flow needs a bit of fixing..

The issue is this:

static __always_inline long __get_user_pages_locked():
{
        if (locked) {
                /* if VM_FAULT_RETRY can be returned, vmas become invalid */
                BUG_ON(vmas);
                /* check caller initialized locked */
                BUG_ON(*locked != 1);
        }


so remote could be written as:

if (gup_flags & FOLL_LONGTERM) {
   if (WARN_ON_ONCE(locked))
        return -EINVAL;
   return __gup_longterm_locked(...)
}

return __get_user_pages_locked(...)

??

Jason