On Sun, Dec 19, 2021 at 12:01:59AM -0800, John Hubbard wrote: > On 12/18/21 22:02, Nadav Amit wrote: > > I found my old messy code for the software-PTE thing. > > > > I see that eventually I decided to hold a pointer to the “extra PTEs” > > of each page in the PMD-page-struct. [ I also implemented the 2-adjacent > > pages approach but this code is long gone. ] > > a) The PMD-page-struct approach won't help as much, because (assuming > that we're using it in an attempt to get a true, perfect pin count), you > are combining the pin counts of a PMD's worth of pages. OTOH...maybe > that actually *is* OK, assuming you don't overflow--except that you can > only answer the "is it dma-pinned?" question at a PMD level. That's a > contradiction of your stated desire above to have very granular control. > > Also, because of not having bit 0 available in page._pt_pad_1, I think > the count would have to be implemented as adding and subtracting 2, > instead of 1 (in order to keep the value even), further reducing the > counter range. I think you misunderstood Nadav's approach. He's talking about making an extra side-allocation per PMD if you're using uffd, and storing extra information in it. I think it's a worthwile approach.
