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

From: Yeoreum Yun <yeoreum.yun@arm.com>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Yeoreum Yun <yeoreum.yun@arm.com>,
	Dave Hansen <dave.hansen@intel.com>,
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Subject: Re: [RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()
Date: Tue, 14 Jul 2026 18:58:57 +0100	[thread overview]
Message-ID: <alZ44XdgTEVFX6jU@e129823.arm.com> (raw)
In-Reply-To: <7cff02d6-d3ce-4315-b667-34911fc6f321@kernel.org>

On Tue, Jul 14, 2026 at 07:29:38PM +0200, David Hildenbrand (Arm) wrote:
> On 7/14/26 18:51, Yeoreum Yun wrote:
> > 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:
> >>>
> >>> 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;
> 
> Do we actually ever get !mm ?

Yes. for the case of s390 use mm anyway for folded check.
> 
> -- 
> Cheers,
> 
> David

-- 
Sincerely,
Yeoreum Yun

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