On Tue, 1 Sep 2020, Maoming (maoming, Cloud Infrastructure Service Product Dept.) wrote: > > > > > In the original process of dma_map/unmap pages for VFIO-devices, to > > > make sure the pages are contiguous, we have to check them one by one. > > > As a result, dma_map/unmap could spend a long time. > > > Using the hugetlb pages, we can avoid this problem. > > > All pages in hugetlb pages are contiguous.And the hugetlb page should > > > not be split.So we can delete the for loops. > > > > I know nothing about VFIO, but I'm surprised that you're paying such attention > > to PageHuge hugetlbfs pages, rather than to PageCompound > > pages: which would also include Transparent Huge Pages, of the traditional > > anonymous kind, or the huge tmpfs kind, or the more general (not necessarily > > pmd-sized) kind that Matthew Wilcox is currently working on. > > > > It's true that hugetlbfs is peculiar enough that whatever you write for it may > > need some tweaks to cover the THP case too, or vice versa; but wouldn't your > > patch be a lot better for covering all cases? > > > > You mention above that "the hugetlb page should not be split": > > perhaps you have been worried that a THP could be split beneath you? > > That used to be a possibility some years ago, but nowadays a THP cannot be > > split while anyone is pinning it with an extra reference. > > > > Hugh > > > > > Thanks for your suggestions. > You mention that a THP cannot be split while anyone is pinning it. > Do you mean the check of can_split_huge_page()(in split_huge_page_to_list())? Partly, yes: but that's just a racy check to avoid doing wasted work in common cases; the more important check comes later in page_ref_freeze(), which either fails, or prevents anyone else taking a new reference to the THP (head or tails) while all the work on splitting is being done. > When we want to pin pages, vfio_pin_pages_remote() always gets a normal page first. > In this case, a THP cannot be split because of the increased reference. > Is this right? Yes. > > Maybe I can optimize for hugetlb pages as a first step, > and then cover all compound pages. Okay. That's definitely a good way to start out - but you may find that the second step leads you to rewrite every line you wrote before! Hugh
