On Thu, Dec 13, 2018 at 7:53 PM John Hubbard <jhubbard@xxxxxxxxxx> 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.
