On Wed, Sep 09, 2020 at 09:01:14AM +0100, Christoph Hellwig wrote: > I really don't think this approach is any good. You workaround > a deficiency in the pin_user_pages API in one particular caller for > one particular use case. RDMA has the same basic issues, this should should not be solved with workarounds in VFIO - a common API would be good > I think you'd rather want either: > > (1) a FOLL_HUGEPAGE flag for the pin_user_pages API family that returns > a single struct page for any kind of huge page, which would also > benefit all kinds of other users rather than adding these kinds of > hacks to vfio. How to use? The VMAs can have mixed page sizes so the caller would have to somehow switch and call twice? Not sure this is faster. > (2) add a bvec version of the API that returns a variable size > "extent" This is the best one, I think.. The IOMMU setup can have multiple page sizes, so having largest contiguous blocks pre-computed should speed that up. vfio should be a win to use a sgl rather than a page list? Especially if we can also reduce the number of pages pinned by only pinning head pages.. > I had started on (2) a while ago, and here is branch with my code (which > is broken and fails test, but might be a start): > > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec > > But for now I wonder if (1) is the better start, which could still be > reused to (2) later. What about some 'pin_user_page_sgl' as a stepping stone? Switching from that point to bvec seems like a smaller step? Jason
