On Thu, Dec 12, 2019 at 9:39 AM Liran Alon <liran.alon@xxxxxxxxxx> wrote: > > > > > On 12 Dec 2019, at 18:54, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > > > On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <liran.alon@xxxxxxxxxx> wrote: > >> > >> > >> > >>> On 11 Dec 2019, at 23:32, Barret Rhoden <brho@xxxxxxxxxx> wrote: > >>> > >>> This change allows KVM to map DAX-backed files made of huge pages with > >>> huge mappings in the EPT/TDP. > >>> > >>> DAX pages are not PageTransCompound. The existing check is trying to > >>> determine if the mapping for the pfn is a huge mapping or not. For > >>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound. > >>> For DAX, we can check the page table itself. > >> > >> For hugetlbfs pages, tdp_page_fault() -> mapping_level() -> host_mapping_level() -> kvm_host_page_size() -> vma_kernel_pagesize() > >> will return the page-size of the hugetlbfs without the need to parse the page-tables. > >> See vma->vm_ops->pagesize() callback implementation at hugetlb_vm_ops->pagesize()==hugetlb_vm_op_pagesize(). > >> > >> Only for pages that were originally mapped as small-pages and later merged to larger pages by THP, there is a need to check for PageTransCompound(). Again, instead of parsing page-tables. > >> > >> Therefore, it seems more logical to me that: > >> (a) If DAX-backed files are mapped as large-pages to userspace, it should be reflected in vma->vm_ops->page_size() of that mapping. Causing kvm_host_page_size() to return the right size without the need to parse the page-tables. > > > > A given dax-mapped vma may have mixed page sizes so ->page_size() > > can't be used reliably to enumerating the mapping size. > > Naive question: Why don’t split the VMA in this case to multiple VMAs with different results for ->page_size()? Filesystems traditionally have not populated ->pagesize() in their vm_operations, there was no compelling reason to go add it and the complexity seems prohibitive. > What you are describing sounds like DAX is breaking this callback semantics in an unpredictable manner. It's not unpredictable. vma_kernel_pagesize() returns PAGE_SIZE. Huge pages in the page cache has a similar issue. > >> (b) If DAX-backed files small-pages can be later merged to large-pages by THP, then the “struct page” of these pages should be modified as usual to make PageTransCompound() return true for them. I’m not highly familiar with this mechanism, but I would expect THP to be able to merge DAX-backed files small-pages to large-pages in case DAX provides “struct page” for the DAX pages. > > > > DAX pages do not participate in THP and do not have the > > PageTransCompound accounting. The only mechanism that records the > > mapping size for dax is the page tables themselves. > > What is the rational behind this? Given that DAX pages can be described with “struct page” (i.e. ZONE_DEVICE), what prevents THP from manipulating page-tables to merge multiple DAX PFNs to a larger page? THP accounting is a function of the page allocator. ZONE_DEVICE pages are excluded from the page allocator. ZONE_DEVICE is just enough infrastructure to support pfn_to_page(), page_address(), and get_user_pages(). Other page allocator services beyond that are not present.
