Re: [PATCH v2] mm/swapops,rmap: remove should-never-be-compiled codes.

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 31 Dec 2025 10:28:25 +1300 Barry Song <21cnbao@gmail.com> wrote:

> On Wed, Dec 31, 2025 at 8:50 AM David Hildenbrand (Red Hat)
> <david@kernel.org> wrote:
> >
> >
> > > -                     break;
> > > +#ifdef CONFIG_NO_PAGE_MAPCOUNT
> > > +             last = atomic_add_negative(-1, &folio->_entire_mapcount);
> > > +             if (level == PGTABLE_LEVEL_PMD && last)
> > > +                     nr_pmdmapped = folio_large_nr_pages(folio);
> > > +             nr = folio_dec_return_large_mapcount(folio, vma);
> > > +             if (!nr) {
> > > +                     /* Now completely unmapped. */
> > > +                     nr = folio_large_nr_pages(folio);
> > > +             } else {
> > > +                     partially_mapped = last &&
> > > +                             nr < folio_large_nr_pages(folio);
> > > +                     nr = 0;
> >
> > The whole code was written to avoid ifdefs.
> 
> The #ifdefs are only for -O0 builds, which are never a
> mainline requirement. However, dropping functions that
> contain nothing but a BUILD_BUG() seems reasonable?

Let's not make -O0 an objective, please.  We often make assumptions
about dead code elimination in order to keep kernel code more pleasing
to read and to maintain.

> If those functions were actually required, the compiler
> would fail anyway. In that case, there doesn’t seem to be
> much value in keeping wrapper functions whose sole purpose
> is to contain a single BUILD_BUG().

I agree with that part of the patch - if the thing only does a
BUILD_BUG() then let's simply remove it.  If somehow the compiler tries
to reference the now-not-present function then we'll get an error message
anyway.