On Tue, May 23, 2023 at 09:16:11PM +0100, David Howells wrote: > I've been poking at it this afternoon, but it doesn't look like it's going to > be straightforward, unfortunately. The mm folks have been withdrawing access > to the pinning API behind the ramparts of the mm/ dir. Further, the dio code > will (I think), under some circumstances, arbitrarily insert the zero_page > into a list of things that are maybe pinned or maybe unpinned, but I can (I > think) also be given a pinned zero_page from the GUP code if the page tables > point to one and a DIO-write is requested - so just doing if page == zero_page > isn't sufficient. Yes. I think the proper workaround is to add a MM helper that just pins a single page and make it available to direct-io.c. It should not be exported and clearly marked to not be used in new code. > What I'd like to do is to make the GUP code not take a ref on the zero_page > if, say, FOLL_DONT_PIN_ZEROPAGE is passed in, and then make the bio cleanup > code always ignore the zero_page. I don't think that'll work, as we can't mix different pin vs get types in a bio. And that's really a good thing. > Something that I noticed is that the dio code seems to wangle to page bits on > the target pages for a DIO-read, which seems odd, but I'm not sure I fully > understand the code yet. I don't understand this sentence.
