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Subject: Re: [PATCH v2 17/39] mm: Fixup places that call pte_mkwrite()
 directly
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20220929222936.14584-18-rick.p.edgecombe@intel.com>
Date:   Mon, 3 Oct 2022 18:56:15 -0700
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References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-18-rick.p.edgecombe@intel.com>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Precedence: bulk

Hopefully I will not waste your time again=E2=80=A6 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@intel.com> =
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 =3D mk_pte(page, vma->vm_page_prot);
> -		if (vma->vm_flags & VM_WRITE)
> -			entry =3D pte_mkwrite(pte_mkdirty(entry));
> +		entry =3D 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 =3D pte_mkdirty(entry);
			entry =3D maybe_mkwrite(entry, vma);
		}