Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE_COW

Borislav Petkov
2023-02-09T14:09:02Z

On Thu, Jan 19, 2023 at 01:22:49PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> The Write=0,Dirty=1 PTE has been used to indicate copy-on-write pages.
> However, newer x86 processors also regard a Write=0,Dirty=1 PTE as a
> shadow stack page. In order to separate the two, the software-defined
> _PAGE_DIRTY is changed to _PAGE_COW for the copy-on-write case, and
> pte_*() are updated to do this.

"In order to separate the two, change the software-defined ..."

From section "2) Describe your changes" in
Documentation/process/submitting-patches.rst:

"Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour."

> +static inline pte_t __pte_mkdirty(pte_t pte, bool soft)
> +{
> + pteval_t dirty = _PAGE_DIRTY;
> +
> + if (soft)
> + dirty |= _PAGE_SOFT_DIRTY;
> +
> + return pte_set_flags(pte, dirty);
> +}

Dunno, do you even need that __pte_mkdirty() helper?

AFAIU, pte_mkdirty() will always set _PAGE_SOFT_DIRTY too so whatever
the __pte_mkdirty() thing needs to do, you can simply do it by foot in
the two callsites.

And this way you won't have the confusion: should I use pte_mkdirty() or
__pte_mkdirty()?

Ditto for the pmd variants.

Otherwise, this is starting to make more sense now.

Thx.

--
Regards/Gruss,
Boris.