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Date:   Mon, 10 May 2021 08:29:32 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [PULL] topic/iomem-mmap-vs-gup
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On Mon, May 10, 2021 at 09:16:58AM +0200, Daniel Vetter wrote:
> > End result: not pulling it, unless somebody can explain to me in small
> > words why I'm wrong and have the mental capacity of a damaged rodent.
> 
> No rodents I think, just more backstory of how this all fits. tldr;
> pin_user_pages is the only safe use of this vb2 userptr thing.

Yes, which is why I advocate for just ripping the follow_pfn path
out entirely.  It could have been used for crazy ad dangerous peer to
peer transfers outside of any infrastructure making it safe, or for
pre-CMA kernel memory carveouts for lage contiguous memory allocations
(which are pretty broken by design as well).  So IMHO the only sensible
thing is to remove this cruft entirely, and if it breaks a currently
working setup (which I think is unlikely) we'll have to make sure it
can work the proper way.