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Date:   Mon, 4 Nov 2019 16:37:02 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Christoph Hellwig <hch@infradead.org>,
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Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN
Message-ID: <20191104203702.GG30938@ziepe.ca>
References: <20191103211813.213227-1-jhubbard@nvidia.com>
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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??

Jason