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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon, 2 Nov 2020 13:56:10 +0100
Message-ID: <CAKMK7uEe5FQuukYU7RhL90ttC9XyWw6wvdQrZ2JpP0jpbYTO6g@mail.gmail.com>
Subject: Re: [PATCH v5 08/15] mm: Add unsafe_follow_pfn
To:     Christoph Hellwig <hch@infradead.org>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "J??r??me Glisse" <jglisse@redhat.com>, Jan Kara <jack@suse.cz>
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On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > that by drivers/modules is together with an mmu_notifier, and that's
> > all _GPL stuff.
>
> I also think it also needs to be renamed to explicitly break any existing
> users out of tree or int the submission queue.

Ok I looked at the mmu notifier locking again and noticed that
mm->subscriptions has its own spinlock. Since there usually shouldn't
be a huge pile of these I think it's feasible to check for the mmu
notifier in follow_pfn. And that would stuff this gap for good. I'll
throw that on top as a final patch and see what people think.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch