Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Dec 13, 2018 at 7:53 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 12/12/18 4:51 PM, Dave Chinner wrote:
> > On Wed, Dec 12, 2018 at 04:59:31PM -0500, Jerome Glisse wrote:
> >> On Thu, Dec 13, 2018 at 08:46:41AM +1100, Dave Chinner wrote:
> >>> On Wed, Dec 12, 2018 at 10:03:20AM -0500, Jerome Glisse wrote:
> >>>> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
> >>>>> On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
> >>>>> So this approach doesn't look like a win to me over using counter in struct
> >>>>> page and I'd rather try looking into squeezing HMM public page usage of
> >>>>> struct page so that we can fit that gup counter there as well. I know that
> >>>>> it may be easier said than done...
> >>>>
>
> Agreed. After all the discussion this week, I'm thinking that the original idea
> of a per-struct-page counter is better. Fortunately, we can do the moral equivalent
> of that, unless I'm overlooking something: Jerome had another proposal that he
> described, off-list, for doing that counting, and his idea avoids the problem of
> finding space in struct page. (And in fact, when I responded yesterday, I initially
> thought that's where he was going with this.)
>
> So how about this hybrid solution:
>
> 1. Stay with the basic RFC approach of using a per-page counter, but actually
> store the counter(s) in the mappings instead of the struct page. We can use
> !PageAnon and page_mapping to look up all the mappings, stash the dma_pinned_count
> there. So the total pinned count is scattered across mappings. Probably still need
> a PageDmaPinned bit.

How do you safely look at page->mapping from the get_user_pages_fast()
path? You'll be racing invalidation disconnecting the page from the
mapping.