linux-kernel - Re: [PATCH v5 08/15] mm: Add unsafe_follow

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Message-ID: <CAKMK7uEe5FQuukYU7RhL90ttC9XyWw6wvdQrZ2JpP0jpbYTO6g@mail.gmail.com>
Date:   Mon, 2 Nov 2020 13:56:10 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        "J??r??me Glisse" <jglisse@...hat.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v5 08/15] mm: Add unsafe_follow_pfn

On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch@...radead.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