memblock: warn when freeing reserved memory before memory map is initialized

When CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, freeing of reserved
memory before the memory map is fully initialized in deferred_init_memmap()
would cause access to uninitialized struct pages and may crash when
accessing spurious list pointers, like was recently discovered during
discussion about memory leaks in x86 EFI code [1].

The trace below is from an attempt to call free_reserved_page() before
page_alloc_init_late():

[    0.076840] BUG: unable to handle page fault for address: ffffce1a005a0788
[    0.078226] #PF: supervisor read access in kernel mode
[    0.078226] #PF: error_code(0x0000) - not-present page
[    0.078226] PGD 0 P4D 0
[    0.078226] Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
[    0.078226] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.12.68-92.123.amzn2023.x86_64 #1
[    0.078226] Hardware name: Amazon EC2 t3a.nano/, BIOS 1.0 10/16/2017
[    0.078226] RIP: 0010:__list_del_entry_valid_or_report+0x32/0xb0
...
[    0.078226]  __free_one_page+0x170/0x520
[    0.078226]  free_pcppages_bulk+0x151/0x1e0
[    0.078226]  free_unref_page_commit+0x263/0x320
[    0.078226]  free_unref_page+0x2c8/0x5b0
[    0.078226]  ? srso_return_thunk+0x5/0x5f
[    0.078226]  free_reserved_page+0x1c/0x30
[    0.078226]  memblock_free_late+0x6c/0xc0

Currently there are not many callers of free_reserved_area() and they all
appear to be at the right timings.

Still, in order to protect against problematic code moves or additions of
new callers add a warning that will inform that reserved pages cannot be
freed until the memory map is fully initialized.

[1] https://lore.kernel.org/all/e5d5a1105d90ee1e7fe7eafaed2ed03bbad0c46b.camel@kernel.crashing.org/

Link: https://patch.msgid.link/20260323074836.3653702-10-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: rppt

mm/internal.h

@@ -1233,7 +1233,17 @@ static inline void vunmap_range_noflush(unsigned long start, unsigned long end)
 #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
 DECLARE_STATIC_KEY_TRUE(deferred_pages);
 
+static inline bool deferred_pages_enabled(void)
+{
+	return static_branch_unlikely(&deferred_pages);
+}
+
 bool __init deferred_grow_zone(struct zone *zone, unsigned int order);
+#else
+static inline bool deferred_pages_enabled(void)
+{
+	return false;
+}
 #endif /* CONFIG_DEFERRED_STRUCT_PAGE_INIT */
 
 void init_deferred_page(unsigned long pfn, int nid);

mm/memblock.c

@@ -900,6 +900,11 @@ static unsigned long __free_reserved_area(phys_addr_t start, phys_addr_t end,
 {
 	unsigned long pages = 0, pfn;
 
+	if (deferred_pages_enabled()) {
+		WARN(1, "Cannot free reserved memory because of deferred initialization of the memory map");
+		return 0;
+	}
+
 	for_each_valid_pfn(pfn, PFN_UP(start), PFN_DOWN(end)) {
 		struct page *page = pfn_to_page(pfn);
 		void *direct_map_addr;

mm/page_alloc.c

@@ -331,11 +331,6 @@ int page_group_by_mobility_disabled __read_mostly;
  */
 DEFINE_STATIC_KEY_TRUE(deferred_pages);
 
-static inline bool deferred_pages_enabled(void)
-{
-	return static_branch_unlikely(&deferred_pages);
-}
-
 /*
  * deferred_grow_zone() is __init, but it is called from
  * get_page_from_freelist() during early boot until deferred_pages permanently
@@ -348,11 +343,6 @@ _deferred_grow_zone(struct zone *zone, unsigned int order)
 	return deferred_grow_zone(zone, order);
 }
 #else
-static inline bool deferred_pages_enabled(void)
-{
-	return false;
-}
-
 static inline bool _deferred_grow_zone(struct zone *zone, unsigned int order)
 {
 	return false;