Jason Gunthorpe <j...@ziepe.ca> writes: > On Wed, Dec 02, 2020 at 08:34:32PM -0500, Pavel Tatashin wrote: >> What I meant is the users of the interface do it incrementally not in >> large chunks. For example: >> >> vfio_pin_pages_remote >> vaddr_get_pfn >> ret = pin_user_pages_remote(mm, vaddr, 1, flags | >> FOLL_LONGTERM, page, NULL, NULL); >> 1 -> pin only one pages at a time > > I don't know why vfio does this, it is why it so ridiculously slow at > least.
Well Alex can correct me, but I went digging and a comment from the first type1 vfio commit says the iommu API didn't promise to unmap subpages of previous mappings, so doing page at a time gave flexibility at the cost of inefficiency. Then 166fd7d94afd allowed the iommu to use larger pages in vfio, but vfio kept pinning pages at a time. I couldn't find an explanation for why that stayed the same. Yesterday I tried optimizing vfio to skip gup calls for tail pages after Matthew pointed out this same issue to me by coincidence last week. Currently debugging, but if there's a fundamental reason this won't work on the vfio side, it'd be nice to know.
