mm/page_ext: init page_ext early if there are no deferred struct pages

page_ext must be initialized after all struct pages are initialized. 
Therefore, page_ext is initialized after page_alloc_init_late(), and can
optionally be initialized earlier via early_page_ext kernel parameter
which as a side effect also disables deferred struct pages.

Allow to automatically init page_ext early when there are no deferred
struct pages in order to be able to use page_ext during kernel boot and
track for example page allocations early.

[pasha.tatashin@soleen.com: fix build with CONFIG_PAGE_EXTENSION=n]
  Link: https://lkml.kernel.org/r/20230118155251.2522985-1-pasha.tatashin@soleen.com
Link: https://lkml.kernel.org/r/20230117204617.1553748-1-pasha.tatashin@soleen.comSigned-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page_ext.h

@@ -29,6 +29,8 @@ struct page_ext_operations {
 	bool need_shared_flags;
 };
 
+extern bool deferred_struct_pages;
+
 #ifdef CONFIG_PAGE_EXTENSION
 
 /*

init/main.c

@@ -855,8 +855,8 @@ static void __init mm_init(void)
 	pgtable_init();
 	debug_objects_mem_init();
 	vmalloc_init();
-	/* Should be run after vmap initialization */
-	if (early_page_ext_enabled())
+	/* If no deferred init page_ext now, as vmap is fully initialized */
+	if (!deferred_struct_pages)
 		page_ext_init();
 	/* Should be run before the first non-init thread is created */
 	init_espfix_bsp();
@@ -1628,7 +1628,7 @@ static noinline void __init kernel_init_freeable(void)
 	padata_init();
 	page_alloc_init_late();
 	/* Initialize page ext after all struct pages are initialized. */
-	if (!early_page_ext_enabled())
+	if (deferred_struct_pages)
 		page_ext_init();
 
 	do_basic_setup();

mm/page_alloc.c

@@ -430,6 +430,8 @@ EXPORT_SYMBOL(nr_online_nodes);
 
 int page_group_by_mobility_disabled __read_mostly;
 
+bool deferred_struct_pages __meminitdata;
+
 #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
 /*
  * During boot we initialize deferred pages on-demand, as needed, but once
@@ -6803,8 +6805,10 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
 		if (context == MEMINIT_EARLY) {
 			if (overlap_memmap_init(zone, &pfn))
 				continue;
-			if (defer_init(nid, pfn, zone_end_pfn))
+			if (defer_init(nid, pfn, zone_end_pfn)) {
+				deferred_struct_pages = true;
 				break;
+			}
 		}
 
 		page = pfn_to_page(pfn);

mm/page_ext.c

@@ -92,7 +92,7 @@ unsigned long page_ext_size;
 static unsigned long total_usage;
 static struct page_ext *lookup_page_ext(const struct page *page);
 
-bool early_page_ext;
+bool early_page_ext __meminitdata;
 static int __init setup_early_page_ext(char *str)
 {
 	early_page_ext = true;