On Thu, Dec 13, 2018 at 10:11:09PM -0800, John Hubbard wrote: > On 12/13/18 9:21 PM, Dan Williams wrote: > > 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. > > > > I don't have an answer for that, so maybe the page->mapping idea is dead already. > > So in that case, there is still one more way to do all of this, which is to > combine ZONE_DEVICE, HMM, and gup/dma information in a per-page struct, and get > there via basically page->private, more or less like this: The page mapcount idea does work to get a pin count. So i believe that this is what should be pursue, if no one wants to try it i will do patches. Anything else is too invasive and requires too much changes. Note that in all the discussion that happened in the mapcount having a separate pin count would not have help one bit nor would it solve the page_mkwrite issue. So we need to audit put_user_page call place and see if they can sleep and call mkwrite without issue. I believe the answer will be yes for many ... maybe all. Cheers, Jérôme
