On Fri, May 08, 2026 at 04:55:17PM +0100, Kiryl Shutsemau (Meta) wrote:
> Preparatory patch. Add the change_protection() primitive that
> userfaultfd RWP will use.
>
> An RWP-protected PTE is PAGE_NONE with the uffd PTE bit set. The
> PROT_NONE half makes the CPU fault on any access; the uffd bit
> distinguishes an RWP fault from a plain mprotect(PROT_NONE) or NUMA
> hinting fault. MM_CP_UFFD_WP and MM_CP_UFFD_RWP share the same PTE
> bit, so the two cannot be used together on the same range.
>
> Two new change_protection() flags:
>
> MM_CP_UFFD_RWP install PAGE_NONE and set the uffd bit
> MM_CP_UFFD_RWP_RESOLVE restore vma->vm_page_prot, clear the uffd bit
>
> Both are wired through change_pte_range(), change_huge_pmd(), and
> hugetlb_change_protection() so anon, shmem, THP, and hugetlb all
> share the same semantics.
>
> Signed-off-by: Kiryl Shutsemau <[email protected]>
> Assisted-by: Claude:claude-opus-4-6
> ---
> include/linux/mm.h | 5 +++++
> include/linux/userfaultfd_k.h | 1 -
> mm/huge_memory.c | 20 ++++++++++++------
> mm/hugetlb.c | 25 ++++++++++++++++------
> mm/mprotect.c | 40 +++++++++++++++++++++++++++++------
> 5 files changed, 71 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3f53d1e978c0..2b65416bb760 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3291,6 +3291,11 @@ int get_cmdline(struct task_struct *task, char
> *buffer, int buflen);
> #define MM_CP_UFFD_WP_RESOLVE (1UL << 3) /* Resolve wp */
> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
> MM_CP_UFFD_WP_RESOLVE)
> +/* Whether this change is for uffd RWP */
> +#define MM_CP_UFFD_RWP (1UL << 4) /* do rwp */
> +#define MM_CP_UFFD_RWP_RESOLVE (1UL << 5) /* Resolve rwp */
Nit: any reason except copy/paset to use different case in "do rwp" and
"Resolve rwp"? ;-)
> +#define MM_CP_UFFD_RWP_ALL (MM_CP_UFFD_RWP | \
> + MM_CP_UFFD_RWP_RESOLVE)
>
> bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
> pte_t pte);
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index fcf308dba311..3725e61a7041 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -397,7 +397,6 @@ static inline bool userfaultfd_huge_pmd_wp(struct
> vm_area_struct *vma,
> return false;
> }
>
> -
> static inline bool userfaultfd_armed(struct vm_area_struct *vma)
> {
> return false;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d88fcccd386d..2537dca63c6c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2665,6 +2665,8 @@ int change_huge_pmd(struct mmu_gather *tlb, struct
> vm_area_struct *vma,
> spinlock_t *ptl;
> pmd_t oldpmd, entry;
> bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> + bool uffd_rwp = cp_flags & MM_CP_UFFD_RWP;
> + bool uffd_rwp_resolve = cp_flags & MM_CP_UFFD_RWP_RESOLVE;
> bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
It looks like uffd_wp* are always ORed with uffd_rwp, we could fold this to
e.g.
bool uffd_prot = cp_flags & (MM_CP_UFFD_WP | MM_CP_UFFD_RWP);
> int ret = 1;
> @@ -2679,11 +2681,18 @@ int change_huge_pmd(struct mmu_gather *tlb, struct
> vm_area_struct *vma,
> return 0;
>
> if (thp_migration_supported() && pmd_is_valid_softleaf(*pmd)) {
> - change_non_present_huge_pmd(mm, addr, pmd, uffd_wp,
> - uffd_wp_resolve);
> + change_non_present_huge_pmd(mm, addr, pmd,
> + uffd_wp || uffd_rwp,
> + uffd_wp_resolve ||
> uffd_rwp_resolve);
> goto unlock;
> }
>
> + /* Already in the desired state */
> + if (prot_numa && pmd_protnone(*pmd))
> + goto unlock;
> + if (uffd_rwp && pmd_protnone(*pmd) && pmd_uffd(*pmd))
> + goto unlock;
> +
> if (prot_numa) {
>
> /*
> @@ -2694,9 +2703,6 @@ int change_huge_pmd(struct mmu_gather *tlb, struct
> vm_area_struct *vma,
> if (is_huge_zero_pmd(*pmd))
> goto unlock;
>
> - if (pmd_protnone(*pmd))
> - goto unlock;
> -
> if (!folio_can_map_prot_numa(pmd_folio(*pmd), vma,
>
> vma_is_single_threaded_private(vma)))
> goto unlock;
> @@ -2725,9 +2731,9 @@ int change_huge_pmd(struct mmu_gather *tlb, struct
> vm_area_struct *vma,
> oldpmd = pmdp_invalidate_ad(vma, addr, pmd);
>
> entry = pmd_modify(oldpmd, newprot);
> - if (uffd_wp)
> + if (uffd_wp || uffd_rwp)
> entry = pmd_mkuffd(entry);
> - else if (uffd_wp_resolve)
> + else if (uffd_wp_resolve || uffd_rwp_resolve)
> /*
> * Leave the write bit to be handled by PF interrupt
> * handler, then things like COW could be properly
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 61cda9992043..63f6b19418b9 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6434,6 +6436,11 @@ long hugetlb_change_protection(struct vm_area_struct
> *vma,
>
> ptep = hugetlb_walk(vma, address, psize);
> if (!ptep) {
> + /*
> + * uffd_wp installs a pte marker on the unpopulated
> + * entry; RWP does not install markers so the
Nit: uffd_rwp
> + * allocation is unnecessary for it.
> + */
> if (!uffd_wp) {
> address |= last_addr_mask;
> continue;
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 8340c8b228c6..23e71f68cf7a 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -216,6 +216,8 @@ static long change_softleaf_pte(struct vm_area_struct
> *vma,
> {
> const bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> const bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> + const bool uffd_rwp = cp_flags & MM_CP_UFFD_RWP;
> + const bool uffd_rwp_resolve = cp_flags & MM_CP_UFFD_RWP_RESOLVE;
And here a single pair of bools should be enough I think.
> softleaf_t entry = softleaf_from_pte(oldpte);
> pte_t newpte;
>
> @@ -256,7 +258,7 @@ static long change_softleaf_pte(struct vm_area_struct
> *vma,
> * to unprotect it, drop it; the next page
> * fault will trigger without uffd trapping.
> */
> - if (uffd_wp_resolve) {
> + if (uffd_wp_resolve || uffd_rwp_resolve) {
> pte_clear(vma->vm_mm, addr, pte);
> return 1;
> }
> @@ -265,9 +267,9 @@ static long change_softleaf_pte(struct vm_area_struct
> *vma,
> newpte = oldpte;
> }
>
> - if (uffd_wp)
> + if (uffd_wp || uffd_rwp)
> newpte = pte_swp_mkuffd(newpte);
> - else if (uffd_wp_resolve)
> + else if (uffd_wp_resolve || uffd_rwp_resolve)
> newpte = pte_swp_clear_uffd(newpte);
>
> if (!pte_same(oldpte, newpte)) {
--
Sincerely yours,
Mike.
