Re: [PATCH 1/2] mm: rmap: make hugetlb pages participate in _nr_pages_mapped

[James Houghton]

On Wed, Mar 8, 2023 at 1:56 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Tue, Mar 07, 2023 at 04:36:51PM -0800, James Houghton wrote:
> > > >       if (likely(!compound)) {
> > > > +             if (unlikely(folio_test_hugetlb(folio)))
> > > > +                     VM_BUG_ON_PAGE(HPageVmemmapOptimized(&folio->page),
> > > > +                                    page);
>
> How about moving folio_test_hugetlb() into the BUG_ON()?
>
>                 VM_BUG_ON_PAGE(folio_test_hugetlb(folio) &&
>                                HPageVmemmapOptimized(&folio->page),
>                                page);
>
> Note that BUG_ON() already contains an "unlikely".

Ok I can do that. It's a little cleaner.

> > > >               first = atomic_inc_and_test(&page->_mapcount);
> > > >               nr = first;
> > > >               if (first && folio_test_large(folio)) {
> > > >                       nr = atomic_inc_return_relaxed(mapped);
> > > >                       nr = (nr < COMPOUND_MAPPED);
> > > >               }
> > > > -     } else if (folio_test_pmd_mappable(folio)) {
> > > > -             /* That test is redundant: it's for safety or to optimize out */
> > >
> > > I 'think' removing this check is OK.  It would seem that the caller
> > > knows if the folio is mappable.  If we want a similar test, we might be
> > > able to use something like:
> > >
> > >         arch_hugetlb_valid_size(folio_size(folio))
> > >
> >
> > Ack. I think leaving the check(s) removed is fine.
>
> Would it still be good to keep that as another BUG_ON()?

Sure, that sounds reasonable to me. I'll add it unless someone disagrees.

As you suggested in your other email, I'll also add a BUG_ON() if we
attempt to do a non-compound mapping of a folio that is larger than
COMPOUND_MAPPED / 2. (Maybe a BUG_ON() in alloc_hugetlb_folio() to
check that the size of the folio we're allocating is less than
COMPOUND_MAPPED / 2 makes sense instead. Just an idea.)

- James