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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 4 Dec 2018 17:00:17 -0800
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Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     John Hubbard <john.hubbard@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Tue, Dec 4, 2018 at 4:58 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 12/4/18 3:03 PM, Dan Williams wrote:
> > On Tue, Dec 4, 2018 at 1:56 PM John Hubbard <jhubbard@nvidia.com> wrote:
[..]
> > Ok, sorry, I mis-remembered. So, you're effectively trying to capture
> > the end of the page pin event separate from the final 'put' of the
> > page? Makes sense.
> >
>
> Yes, that's it exactly.
>
> >> I was not able to actually find any place where a single additional page
> >> bit would help our situation, which is why this still uses LRU fields for
> >> both the two bits required (the RFC [1] still applies), and the dma_pinned_count.
> >
> > Except the LRU fields are already in use for ZONE_DEVICE pages... how
> > does this proposal interact with those?
>
> Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
> use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
> way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
>
> If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole
> LRU field approach is unusable.

Unfortunately, the entire motivation for ZONE_DEVICE was to support
get_user_pages() for persistent memory.