Re: [PATCH v4 6/7] mm/x86: Add missing pud helpers

Thomas Gleixner
2024-08-07T22:37:23Z

On Wed, Aug 07 2024 at 15:48, Peter Xu wrote:
> These new helpers will be needed for pud entry updates soon. Introduce
> these helpers by referencing the pmd ones. Namely:
>
> - pudp_invalidate()
> - pud_modify()

Zero content about what these helpers do and why they are needed. That's
not how it works, really.

> +static inline pud_t pud_mkinvalid(pud_t pud)
> +{
> + return pfn_pud(pud_pfn(pud),
> + __pgprot(pud_flags(pud) & ~(_PAGE_PRESENT|_PAGE_PROTNONE)));

100 characters...

> +}
> +
> static inline u64 flip_protnone_guard(u64 oldval, u64 val, u64 mask);
>
> static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> @@ -834,14 +840,8 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> pmd_result = __pmd(val);
>
> /*
> - * To avoid creating Write=0,Dirty=1 PMDs, pte_modify() needs to avoid:
> - * 1. Marking Write=0 PMDs Dirty=1
> - * 2. Marking Dirty=1 PMDs Write=0
> - *
> - * The first case cannot happen because the _PAGE_CHG_MASK will filter
> - * out any Dirty bit passed in newprot. Handle the second case by
> - * going through the mksaveddirty exercise. Only do this if the old
> - * value was Write=1 to avoid doing this on Shadow Stack PTEs.
> + * Avoid creating shadow stack PMD by accident. See comment in
> + * pte_modify().

The changelog is utterly silent about this comment update.

> */
> if (oldval & _PAGE_RW)
> pmd_result = pmd_mksaveddirty(pmd_result);
> @@ -851,6 +851,29 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> return pmd_result;
> }
>
> +static inline pud_t pud_modify(pud_t pud, pgprot_t newprot)
> +{
> + pudval_t val = pud_val(pud), oldval = val;
> + pud_t pud_result;
> +
> + val &= _HPAGE_CHG_MASK;
> + val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
> + val = flip_protnone_guard(oldval, val, PHYSICAL_PUD_PAGE_MASK);
> +
> + pud_result = __pud(val);
> +
> + /*
> + * Avoid creating shadow stack PUD by accident. See comment in
> + * pte_modify().
> + */
> + if (oldval & _PAGE_RW)
> + pud_result = pud_mksaveddirty(pud_result);
> + else
> + pud_result = pud_clear_saveddirty(pud_result);
> +
> + return pud_result;
> +}
> +
> /*
> * mprotect needs to preserve PAT and encryption bits when updating
> * vm_page_prot
> @@ -1389,10 +1412,26 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> }
> #endif
>
> +static inline pud_t pudp_establish(struct vm_area_struct *vma,
> + unsigned long address, pud_t *pudp, pud_t pud)

Random line break alignment.... See documentation.

> +{
> + page_table_check_pud_set(vma->vm_mm, pudp, pud);
> + if (IS_ENABLED(CONFIG_SMP)) {
> + return xchg(pudp, pud);
> + } else {
> + pud_t old = *pudp;
> + WRITE_ONCE(*pudp, pud);

Lacks a newline between variable declaration and code.

But seriously, why optimizing for !SMP? That's a pointless exercise and
a guarantee for bitrot.

> + return old;
> + }
> +}
> +
> #define __HAVE_ARCH_PMDP_INVALIDATE_AD
> extern pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,
> unsigned long address, pmd_t *pmdp);
>
> +pud_t pudp_invalidate(struct vm_area_struct *vma, unsigned long address,
> + pud_t *pudp);

While 'extern' is not required, please keep the file style consistent
and use the 100 characters...

> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -641,6 +641,18 @@ pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,
> }
> #endif
>
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
> + defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
> +pud_t pudp_invalidate(struct vm_area_struct *vma, unsigned long address,
> + pud_t *pudp)
> +{
> + VM_WARN_ON_ONCE(!pud_present(*pudp));
> + pud_t old = pudp_establish(vma, address, pudp, pud_mkinvalid(*pudp));
> + flush_pud_tlb_range(vma, address, address + HPAGE_PUD_SIZE);
> + return old;

Your keyboard clearly lacks a newline key ...

Thanks,

tglx