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

[David Hildenbrand <david@redhat.com>]

On 14.03.24 02:25, Matthew Wilcox (Oracle) wrote:
> The current folio_test_hugetlb() can be fooled by a concurrent folio split
> into returning true for a folio which has never belonged to hugetlbfs.
> This can't happen if the caller holds a refcount on it, but we have a
> few places (memory-failure, compaction, procfs) which do not and should
> not take a speculative reference.
> 
> Since hugetlb pages do not use individual page mapcounts (they are always
> fully mapped and use the entire_mapcount field to record the number of
> mappings), the PageType field is available for use with only a minor
> modification to page_mapcount() to ignore the value in this field for
> hugetlb pages.  Other parts of the kernel which examine page->_mapcount
> directly already know about page_has_type() (eg procfs, dump_page()).
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---

Should we include this hunk:

diff --git a/mm/debug.c b/mm/debug.c
index c1c1a6a484e4c..f521e17150a10 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -63,7 +63,8 @@ static void __dump_folio(struct folio *folio, struct page *page,
          * encode own info, and we must avoid calling page_folio() again.
          */
         if (!folio_test_slab(folio)) {
-               mapcount = atomic_read(&page->_mapcount) + 1;
+               if (!folio_test_hugetlb(folio))
+                       mapcount = atomic_read(&page->_mapcount) + 1;
                 if (folio_test_large(folio))
                         mapcount += folio_entire_mapcount(folio);
         }

Apart from that, LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb