mm: improve dumping of mapcount and page_type

For pages that have a page_type, set the mapcount to 0, which will reduce
the confusion in people reading page dumps ("Why does this page have a
mapcount of -128?").  Now that hugetlbfs is a page_type, read the
entire_mapcount for any large folio; this is fine for all folios as no
user reuses the entire_mapcount field.

For pages which do not have a page type, do not print it to reduce
clutter.

Link: https://lkml.kernel.org/r/20240321142448.1645400-9-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/debug.c b/mm/debug.c
index c1c1a6a484e4c07606129ef2731a895fd4ddce41..e8a96b8b71973bfdeac6c7e1eb2b6c12efeb3170 100644 (file)
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -55,18 +55,14 @@ static void __dump_folio(struct folio *folio, struct page *page,
                unsigned long pfn, unsigned long idx)
 {
        struct address_space *mapping = folio_mapping(folio);
-       int mapcount = 0;
+       int mapcount = atomic_read(&page->_mapcount) + 1;
        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)) {
-               mapcount = atomic_read(&page->_mapcount) + 1;
-               if (folio_test_large(folio))
-                       mapcount += folio_entire_mapcount(folio);
-       }
+       /* Open-code page_mapcount() to avoid looking up a stale folio */
+       if (mapcount < 0)
+               mapcount = 0;
+       if (folio_test_large(folio))
+               mapcount += folio_entire_mapcount(folio);
 
        pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
                        folio_ref_count(folio), mapcount, mapping,
@@ -99,7 +95,8 @@ static void __dump_folio(struct folio *folio, struct page *page,
         */
        pr_warn("%sflags: %pGp%s\n", type, &folio->flags,
                is_migrate_cma_folio(folio, pfn) ? " CMA" : "");
-       pr_warn("page_type: %pGt\n", &folio->page.page_type);
+       if (page_has_type(&folio->page))
+               pr_warn("page_type: %pGt\n", &folio->page.page_type);
 
        print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
                        sizeof(unsigned long), page,