Re: [PATCH] s390/mm: Add missing ctor/dtor on page table upgrade

Alexander Gordeev
2025-01-24T08:25:22Z

On Fri, Jan 24, 2025 at 08:58:07AM +0100, Kevin Brodsky wrote:

Kevin,
...
> Thank you for putting together this patch! I was completely unaware of
> this "upgrade" path on s390.
...
> > diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
> > index a4e761902093..d33f55b7ee98 100644
> > --- a/arch/s390/mm/pgalloc.c
> > +++ b/arch/s390/mm/pgalloc.c
> > @@ -88,12 +88,14 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
> > if (unlikely(!p4d))
> > goto err_p4d;
> > crst_table_init(p4d, _REGION2_ENTRY_EMPTY);
> > + pagetable_p4d_ctor(virt_to_ptdesc(p4d));
>
> This block seems to be equivalent to p4d_alloc_one(), maybe it would be
> preferable to just call p4d_alloc_one() here to avoid further mismatches
> in the future (and reduce duplication)?

Yes, I wanted to do exactly that (together with p4d_free() you noticed),
but then the whole thing with open coded pagetable_pgd_ctor() below gets
inconsistent. I would prefer to keep it this way as of now.

> > }
> > if (end > _REGION1_SIZE) {
> > pgd = crst_table_alloc(mm);
> > if (unlikely(!pgd))
> > goto err_pgd;
> > crst_table_init(pgd, _REGION1_ENTRY_EMPTY);
> > + pagetable_pgd_ctor(virt_to_ptdesc(pgd));
>
> I was hoping this might be equivalent to pgd_alloc() but it does not
> include a call to crst_table_init(). Since adding it would be apparently
> undesirable (having read the other thread), it seems reasonable to add
> the explicit constructor call.

We were thinking about a follow-up cleanup that addresses it all, but this
patch is a targeted fix to catch up your and Qi Zheng series in the still
open merge window.

> > }
> >
> > spin_lock_bh(&mm->page_table_lock);
> > @@ -130,6 +132,7 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
> > return 0;
> >
> > err_pgd:
> > + pagetable_dtor(virt_to_ptdesc(p4d));
> > crst_table_free(mm, p4d);
>
> Similarly, this could be a call to p4d_free().
>
> - Kevin
>
> > err_p4d:
> > return -ENOMEM;

Thanks for the review!