> On 12 Dec 2019, at 19:59, Dan Williams <[email protected]> wrote: > > On Thu, Dec 12, 2019 at 9:39 AM Liran Alon <[email protected]> wrote: >> >> >> >>> On 12 Dec 2019, at 18:54, Dan Williams <[email protected]> wrote: >>> >>> On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <[email protected]> wrote: >>>> >>>> >>>> >>>>> On 11 Dec 2019, at 23:32, Barret Rhoden <[email protected]> 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.
I understand. Though this is technical debt that breaks ->page_size() semantics which might cause a complex bug some day... > >> 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. Of course. :) I meant it may be unexpected by the caller. > Huge > pages in the page cache has a similar issue. Ok. I haven’t known that. Thanks for the explanation. > >>>> (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. Ok. _______________________________________________ Linux-nvdimm mailing list -- [email protected] To unsubscribe send an email to [email protected]
