On Mon, Jul 19, 2021 at 09:21:18PM +0200, David Hildenbrand wrote: > Hi Peter, Hi, David, > > I just stumbled over copy_page_range() optimization > > /* > * Don't copy ptes where a page fault will fill them correctly. > * Fork becomes much lighter when there are big shared or private > * readonly mappings. The tradeoff is that copy_page_range is more > * efficient than faulting. > */ > if (!(src_vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) && > !src_vma->anon_vma) > return 0; > > IIUC, that means you'll not copy the WP bits for shmem and, > therefore, lose them during fork. Good point. I think the fix shouldn't be hard - we can also skip this if dst_vma->vm_flags has VM_UFFD_WP set (that means UFFD_FEATURE_EVENT_FORK is enabled too). But I'll check a bit into page copy later to make sure it works (maybe I'll add a small test case too). Thanks! -- Peter Xu
