Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory

Kiryl Shutsemau
2026-04-22T09:27:05Z

On Tue, Apr 21, 2026 at 03:33:27PM +0100, Kiryl Shutsemau wrote:
> > 3) Some other stuff needs a second thought, like
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 8e7dc2c6ee738..08fc18f1290d4 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -695,7 +695,8 @@ static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
> > /* ... and a write-fault isn't required for other reasons. */
> > if (pmd_needs_soft_dirty_wp(vma, pmd))
> > return false;
> > - return !userfaultfd_huge_pmd_wp(vma, pmd);
> > + return !userfaultfd_huge_pmd_wp(vma, pmd) &&
> > + !userfaultfd_huge_pmd_rwp(vma, pmd);
> > }
> >
> > How can a pte be writable and prot_none at the same time? Maybe just confused AI
> > output that you should carefully double check before sending that out officially.
>
> Note that this path is for !pmd_write() case to begin with. It serves
> FOLL_FORCE case. I believe this check is correct: we don't want to allow
> to write to such pages even with FOLL_FORCE.
>
> But looking around, I missed gup_can_follow_protnone() modification. It
> has to return false for RWP.

With gup_can_follow_protnone() fixed, the checks in
can_follow_write_pmd/pte() are redundant. Will drop them.

--
Kiryl Shutsemau / Kirill A. Shutemov