Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Christoph Hellwig]

On Wed, Jan 30, 2019 at 04:18:48AM +0000, Jason Gunthorpe wrote:
> Every attempt to give BAR memory to struct page has run into major
> trouble, IMHO, so I like that this approach avoids that.

Way less problems than not having struct page for doing anything
non-trivial.  If you map the BAR to userspace with remap_pfn_range
and friends the mapping is indeed very simple.  But any operation
that expects a page structure, which is at least everything using
get_user_pages won't work.

So you can't do direct I/O to your remapped BAR, you can't create MRs
on it, etc, etc.