Re: [PATCH v2 6/6] mm: Introduce ctor/dtor at PGD level

[Alexander Gordeev <agordeev@linux.ibm.com>]

On Wed, Jan 22, 2025 at 08:49:54AM +0100, Heiko Carstens wrote:
> > >  static inline pgd_t *pgd_alloc(struct mm_struct *mm)
> > >  {
> > > -	return (pgd_t *) crst_table_alloc(mm);
> > > +	unsigned long *table = crst_table_alloc(mm);
> > > +
> > > +	if (!table)
> > > +		return NULL;
> > 
> > I do not know status of this series, but FWIW, this call is missed:
> > 
> > 	crst_table_init(table, _REGION1_ENTRY_EMPTY); 
> 
> Why is that missing?

Because the follow-up pagetable_pgd_ctor() is called against uninitialized
page table, while other pagetable_pXd_ctor() variants are called against
initialized one. I could imagine complications as result of that.

Whether Region1 table is the right choice is a big question though, as you
noticed below.

> A pgd table can be a Region1, Region2, or Region3 table. The only caller of
> this function is mm_init() via mm_alloc_pgd(); and right after mm_alloc_pgd()
> there is a call to init_new_context() which will initialize the pgd correctly.

init_new_context() is in a way a constructor as well, so whole thing looks odd
to me. But I do not immedeately see a better way :(

> I guess what really gets odd, and might be broken (haven't checked yet) is
> what happens on dynamic upgrade of page table levels (->crst_table_upgrade()).

Hmm, that is a good point.

> With that a pgd may become a pud, and with that we get an imbalance with
> the ctor/dtor calls for the various page table levels when they get freed

The ctor/dtor mismatch should not be a problem, as pagetable_pgd|p4d|pud_ctor()
are the same and there is one pagetable_dtor() for all top levels as of now.
But if it ever comes to separate implementations, then we are in the world
of pain.

> again. Plus, at first glance, it looks also broken that we have open-coded
> crst_alloc() calls instead of using the "proper" page table allocation API
> within crst_table_upgrade(), which again would cause an imbalance.

This is a good point too.

Many thanks!