Re: [PATCH 1/2] mm: Turn folio_test_hugetlb into a PageType

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 15, 2024 at 02:19:10PM +0800, Miaohe Lin wrote:
> On 2024/3/14 22:33, Matthew Wilcox wrote:
> > I actually want to do a bit more here ...
> > 
> > +++ b/mm/debug.c
> > @@ -58,15 +58,10 @@ static void __dump_folio(struct folio *folio, struct page *page,
> >         int mapcount = 0;
> >         char *type = "";
> > 
> > -       /*
> > -        * page->_mapcount space in struct page is used by slab pages to
> > -        * encode own info, and we must avoid calling page_folio() again.
> > -        */
> > -       if (!folio_test_slab(folio)) {
> > +       if (!page_has_type(page)) {
> >                 mapcount = atomic_read(&page->_mapcount) + 1;
> > -               if (folio_test_large(folio))
> > -                       mapcount += folio_entire_mapcount(folio);
> > -       }
> > +       if (folio_test_large(folio))
> > +               mapcount += folio_entire_mapcount(folio);
> 
> Why folio_test_large is not within the "if (!page_has_type(page))" block? I think a slab page could also be large folio.
> Or am I miss something?

Slab pages don't use the first tail page so folio_entire_mapcount will
be 0 for them.  And we don't want the folio_test_large() to be gated by
page_has_type() because hugetlb pages (after this patch) will be in the
page_has_type() category.  So for hugetlb pages, we want to not read
page->mapcount (leaving mapcount at zero), but we do want to read the
entire_mapcount.