Re: [PATCH 17/35] mm: Fixup places that call pte_mkwrite() directly

[Dave Hansen]

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.