On 1/30/22 13:18, Rick Edgecombe wrote: > - do_anonymous_page() and migrate_vma_insert_page() check VM_WRITE directly > and call pte_mkwrite(), which is the same as maybe_mkwrite(). Change > them to maybe_mkwrite(). Those look OK. > - In do_numa_page(), if the numa entry was writable, then pte_mkwrite() > is called directly. Fix it by doing maybe_mkwrite(). Make the same > changes to do_huge_pmd_numa_page(). This is another "what", not "why" changelog. This change puzzles me. *Why* is this needed? It sounds like pte_mkwrite() doesn't work for shadow stack PTEs. Let's say that explicitly. I also this this is ab/misuse of maybe_mkwrite(). The shadow stack VMA *REQUIRES* PTEs with Dirty=1. There's no *maybe* about it. The rest of this is essentially a hack to get VM_SHADOW_STACK-required bits into the PTE. We have a place where we store those VMA-required bits: vma->vm_page_prot. Look at how we store the pkey bits in there for instance. Let's say we set _PAGE_DIRTY in vma->vm_page_prot. We'd come into do_anonymous_page() for instance and do this: > entry = mk_pte(page, vma->vm_page_prot); <--- PTE is Write=0,Dirty=1 Yay! > entry = pte_sw_mkyoung(entry); > if (vma->vm_flags & VM_WRITE) <--- False, skip the pte_mkwrite() > entry = pte_mkwrite(pte_mkdirty(entry)); In other words, it "just works" because shadow stack VMAs don't have VM_WRITE set. I think the other VM_WRITE checks would be fine too, although I'm unsure about the change_page_attr() one.
