> On 12 Dec 2019, at 21:55, Barret Rhoden <brho@xxxxxxxxxx> wrote: > > Hi - > > On 12/12/19 1:49 PM, Liran Alon wrote: >>> On 12 Dec 2019, at 20:47, Liran Alon <liran.alon@xxxxxxxxxx> wrote: >>> >>> >>> >>>> On 12 Dec 2019, at 20:22, 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. >>> >>> This change isn’t only relevant for TDP. It also affects when KVM use shadow-paging. >>> See how FNAME(page_fault)() calls transparent_hugepage_adjust(). > > Cool, I'll drop references to the EPT/TDP from the commit message. > >>>> DAX pages are not PageTransCompound. The existing check is trying to >>>> determine if the mapping for the pfn is a huge mapping or not. >>> >>> I would rephrase “The existing check is trying to determine if the pfn >>> is mapped as part of a transparent huge-page”. > > Can do. > >>> >>>> For >>>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound. >>> >>> This is not related to hugetlbfs but rather THP. > > I thought that PageTransCompound also returned true for hugetlbfs (based off of comments in page-flags.h). Though I do see the comment about the 'level == PT_PAGE_TABLE_LEVEL' check excluding hugetlbfs pages. > > Anyway, I'll remove the "e.g. hugetlbfs" from the commit message. > >>> >>>> For DAX, we can check the page table itself. >>>> >>>> Note that KVM already faulted in the page (or huge page) in the host's >>>> page table, and we hold the KVM mmu spinlock. We grabbed that lock in >>>> kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq. >>>> >>>> Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx> >>> >>> I don’t think the right place to change for this functionality is transparent_hugepage_adjust() >>> which is meant to handle PFNs that are mapped as part of a transparent huge-page. >>> >>> For example, this would prevent mapping DAX-backed file page as 1GB. >>> As transparent_hugepage_adjust() only handles the case (level == PT_PAGE_TABLE_LEVEL). >>> >>> As you are parsing the page-tables to discover the page-size the PFN is mapped in, >>> I think you should instead modify kvm_host_page_size() to parse page-tables instead >>> of rely on vma_kernel_pagesize() (Which relies on vma->vm_ops->pagesize()) in case >>> of is_zone_device_page(). >>> The main complication though of doing this is that at this point you don’t yet have the PFN >>> that is retrieved by try_async_pf(). So maybe you should consider modifying the order of calls >>> in tdp_page_fault() & FNAME(page_fault)(). >>> >>> -Liran >> Or alternatively when thinking about it more, maybe just rename transparent_hugepage_adjust() >> to not be specific to THP and better handle the case of parsing page-tables changing mapping-level to 1GB. >> That is probably easier and more elegant. > > I can rename it to hugepage_adjust(), since it's not just THP anymore. Sounds good. > > I was a little hesitant to change the this to handle 1 GB pages with this patchset at first. I didn't want to break the non-DAX case stuff by doing so. Why would it affect non-DAX case? Your patch should just make hugepage_adjust() to parse page-tables only in case is_zone_device_page(). Otherwise, page tables shouldn’t be parsed. i.e. THP merged pages should still be detected by PageTransCompoundMap(). > > Specifically, can a THP page be 1 GB, and if so, how can you tell? If you can't tell easily, I could walk the page table for all cases, instead of just zone_device(). I prefer to walk page-tables only for is_zone_device_page(). > > I'd also have to drop the "level == PT_PAGE_TABLE_LEVEL" check, I think, which would open this up to hugetlbfs pages (based on the comments). Is there any reason why that would be a bad idea? KVM already supports mapping 1GB hugetlbfs pages. As level is set to PUD-level by tdp_page_fault()->mapping_level()->host_mapping_level()->kvm_host_page_size()->vma_kernel_pagesize(). As VMA which is mmap of hugetlbfs sets vma->vm_ops to hugetlb_vm_ops() where hugetlb_vm_op_pagesize() will return appropriate page-size. Specifically, I don’t think THP ever merges small pages to 1GB pages. I think this is why transparent_hugepage_adjust() checks PageTransCompoundMap() only in case level == PT_PAGE_TABLE_LEVEL. I think you should keep this check in the case of !is_zone_device_page(). -Liran > > Thanks, > > Barret >
