Re: [PATCH RFC v2 13/20] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Wed, Jul 22, 2026 at 01:20:39PM -0700, Dave Hansen wrote:
> On 7/22/26 10:37, Yeoreum Yun wrote:
> >> On 7/22/26 08:30, Yeoreum Yun wrote:
> >>>  struct pg_state {
> >>>  	struct ptdump_state ptdump;
> >>> +	int first_level;
> >>>  	int level;
> >>>  	pgprotval_t current_prot;
> >>>  	pgprotval_t effective_prot;
> >> There is zero reason on x86 to hold this value in a data structure. It
> >> is 100% determined at boot time or earlier. Pre-5-level-paging it's a
> >> compile-time constant.
> >>
> >> On x86, it never varies per-mm.
> > However, mm_pXd_folded() requires to mm for other architecture like
> > s390. might save the mm instead of first_level and calls the
> > ptdump_pt_level_first() with static inline version would make the
> > constant comparison. So it requires to save *mm* structure in here.
> 
> I'm having a really hard time parsing that.
> 
> I think you're trying to say that the effective_prot_p*() functions need
> to know the first level but they don't (today) need the mm_struct. If
> they don't get the (calculated) first_level passed in, they need the
> mm_struct instead.
> 
> I think you're arguing 'pg_state' needs a ->first_level or a ->mm.
> Having a pg_state->mm doesn't seem bad to me at all.
> 
> But, it's also a little bit silly. It would not be rocket science to
> pass an mm_struct down to the effective_prot_p*() functions. It makes a
> whole lot of sense to me for a page table walking function to need
> metadata from the mm_struct to walk correctly.

Yes. I mean to add pg_state->mm like:

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 2afa7a23340e..aaf00f0c6624 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -38,6 +38,7 @@ struct pg_state {
        bool check_wx;
        unsigned long wx_pages;
        struct seq_file *seq;
+       struct mm_struct *mm;
 };

 struct addr_marker {
@@ -254,7 +255,7 @@ static void effective_prot(struct ptdump_state *pt_st, int level, u64 val)
        pgprotval_t prot = val & PTE_FLAGS_MASK;
        pgprotval_t effective;

-       if (level > 0) {
+       if (level > pgtable_first_level(st->mm)) {
                pgprotval_t higher_prot = st->prot_levels[level - 1];

                effective = (higher_prot & prot & (_PAGE_USER | _PAGE_RW)) |
@@ -452,7 +453,8 @@ bool ptdump_walk_pgd_level_core(struct seq_file *m,
                .level = -1,
                .to_dmesg       = dmesg,
                .check_wx       = checkwx,
-               .seq            = m
+               .seq            = m,
+               .mm             = mm,
        };

        ptdump_walk_pgd(&st.ptdump, mm, pgd);
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 8c093c119e5a..6e7d0580db99 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -2490,4 +2490,15 @@ pgprot_t vm_get_page_prot(vm_flags_t vm_flags)                           \
 }                                                                      \
 EXPORT_SYMBOL(vm_get_page_prot);

+static inline int pgtable_first_level(struct mm_struct *mm)
+{
+       if (mm_pmd_folded(mm))
+               return 3;
+       if (mm_pud_folded(mm))
+               return 2;
+       if (mm_p4d_folded(mm))
+               return 1;
+       return 0;
+}
+
 #endif /* _LINUX_PGTABLE_H */

-- 
Sincerely,
Yeoreum Yun