[RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

[Yeoreum Yun]

On Tue, Jul 14, 2026 at 05:08:24PM +0100, Yeoreum Yun wrote:
> On Tue, Jul 14, 2026 at 07:35:19AM -0700, Dave Hansen wrote:
> > On 7/14/26 04:40, Yeoreum Yun wrote:
> > > @@ -254,22 +255,8 @@ static void effective_prot(struct ptdump_state *pt_st, int level, u64 val)
> > >         struct pg_state *st = container_of(pt_st, struct pg_state, ptdump);
> > >         pgprotval_t prot = val & PTE_FLAGS_MASK;
> > >         pgprotval_t effective;
> > > -       bool first_level = false;
> > > 
> > > -       /* Ignore folded levels ... */
> > > -       if (((level == 0) && mm_p4d_folded(st->mm)) ||
> > > -           ((level == 1) && mm_pud_folded(st->mm)) ||
> > > -           ((level == 2) && mm_pmd_folded(st->mm)))
> > > -               return;
> > > -
> > > -       /* ... and make the actual first level remember the protection. */
> > > -       if (((level == 0)) ||
> > > -           ((level == 1) && mm_p4d_folded(st->mm)) ||
> > > -           ((level == 2) && mm_pud_folded(st->mm)) ||
> > > -           ((level == 3) && mm_pmd_folded(st->mm)))
> > > -               first_level = true;
> > > -
> > > -       if (!first_level) {
> > > +       if (first_level > st->first_level) {
> > >                 pgprotval_t higher_prot = st->prot_levels[level - 1];
> > > 
> > >                 effective = (higher_prot & prot & (_PAGE_USER | _PAGE_RW)) |
> > > @@ -471,6 +458,15 @@ bool ptdump_walk_pgd_level_core(struct seq_file *m,
> > >                 .seq            = m
> > >         };
> > > 
> > > +       if (mm_pmd_folded (mm))
> > > +               st->first_level = 3;
> > > +       else if (mm_pud_folded (mm))
> > > +               st->first_level = 2;
> > > +       else if (mm_p4d_folded (mm))
> > > +               st->first_level = 1;
> > > +       else
> > > +               st->first_level = 0;
> > > +
> > >         ptdump_walk_pgd(&st.ptdump, mm, pgd);
> > 
> > This is indeed an improvement and a step in the right direction! Thanks
> > for looking at this.
> > 
> > But one of my test for whether it's good x86 code is whether there's any
> > actually x86-specific logic in it. Isn't this basically a translation
> > between the integer level number and whether it is folded?
> > 
> > That seems like a common helper that more than one arch could use. Could
> > this be stuck in a helper so that all arch/x86 has to do is:
> > 
> > 	if (mm_pt_level_folded(mm, level))
> > 		return;
> 
> Yes. at least the level semantic in ptdump (pgd is level 0,
> p4d is level 1, ...) is same to all archs where use ptdump.
> So It seems reasonable to add this common helper in ptdump.c
> 

Furthermore, the effective_prot() need to require to identify whether
it's the first pgtable to prevent the inheriting from dummy value.
So I think it seems to make a ptdump_pt_level_first() like:

diff --git a/mm/ptdump.c b/mm/ptdump.c
index 973020000096c..2ec5700e4be5e 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -190,6 +190,23 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd)
        st->note_page_flush(st);
 }

+bool pdtump_pt_level_first(struct mm_struct *mm, int level)
+{
+       if (!mm || level > CONFIG_PGTABLE_LEVELS)
+               return false;
+
+       if (mm_pmd_folded(mm) && level == 3)
+               return true;
+       if (mm_pud_folded(mm) && level == 2)
+               return true;
+       if (mm_p4d_folded(mm) && level == 1)
+               return true;
+       if (level == 0)
+               return true;
+
+       return false;
+}

-- 
Sincerely,
Yeoreum Yun