Re: [PATCH v2 17/39] mm: Fixup places that call pte_mkwrite() directly

[Nadav Amit]

Hopefully I will not waste your time again… If it has been discussed in the
last 26 iterations, just tell me and ignore.

On Sep 29, 2022, at 3:29 PM, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> wrote:

> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -606,8 +606,7 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
> 			goto abort;
> 		}
> 		entry = mk_pte(page, vma->vm_page_prot);
> -		if (vma->vm_flags & VM_WRITE)
> -			entry = pte_mkwrite(pte_mkdirty(entry));
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 	}

This is not exactly the same logic. You might dirty read-only pages since
you call pte_mkdirty() unconditionally. It has been known not to be very
robust (e.g., dirty-COW and friends). Perhaps it is not dangerous following
some recent enhancements, but why do you want to take the risk?

Instead, although it might seem redundant, the compiler will hopefully would
make it efficient:

		if (vma->vm_flags & VM_WRITE) {
			entry = pte_mkdirty(entry);
			entry = maybe_mkwrite(entry, vma);
		}