mm/page_alloc: fix race condition in unaccepted memory handling

The page allocator tracks the number of zones that have unaccepted memory
using static_branch_enc/dec() and uses that static branch in hot paths to
determine if it needs to deal with unaccepted memory.

Borislav and Thomas pointed out that the tracking is racy: operations on
static_branch are not serialized against adding/removing unaccepted pages
to/from the zone.

Sanity checks inside static_branch machinery detects it:

WARNING: CPU: 0 PID: 10 at kernel/jump_label.c:276 __static_key_slow_dec_cpuslocked+0x8e/0xa0

The comment around the WARN() explains the problem:

/*
* Warn about the '-1' case though; since that means a
* decrement is concurrent with a first (0->1) increment. IOW
* people are trying to disable something that wasn't yet fully
* enabled. This suggests an ordering problem on the user side.
*/

The effect of this static_branch optimization is only visible on
microbenchmark.

Instead of adding more complexity around it, remove it altogether.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kirill A. Shutemov <[e-mail]>
Fixes: dcdfdd40fa82 ("mm: Add support for unaccepted memory")
Link: https://lore.kernel.org/all/20250506092445.GBaBnVXXyvnazly6iF@fat_crate.local
Reported-by: Borislav Petkov <[e-mail]>
Tested-by: Borislav Petkov (AMD) <[e-mail]>
Reported-by: Thomas Gleixner <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Brendan Jackman <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: <[e-mail]> [6.5+]
Signed-off-by: Andrew Morton <[e-mail]>

mm/internal.h

···
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 void accept_page(struct page *page);
-void unaccepted_cleanup_work(struct work_struct *work);
 #else /* CONFIG_UNACCEPTED_MEMORY */
 static inline void accept_page(struct page *page)
 {

mm/mm_init.c

···
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 	INIT_LIST_HEAD(&zone->unaccepted_pages);
-	INIT_WORK(&zone->unaccepted_cleanup, unaccepted_cleanup_work);
 #endif
 }
 

mm/page_alloc.c

···
 
 #ifdef CONFIG_UNACCEPTED_MEMORY
 
-/* Counts number of zones with unaccepted pages. */
-static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
-
 static bool lazy_accept = true;
-
-void unaccepted_cleanup_work(struct work_struct *work)
-{
-	static_branch_dec(&zones_with_unaccepted_pages);
-}
 
 static int __init accept_memory_parse(char *p)
 {
···
 static void __accept_page(struct zone *zone, unsigned long *flags,
 			  struct page *page)
 {
-	bool last;
-
 	list_del(&page->lru);
-	last = list_empty(&zone->unaccepted_pages);
-
 	account_freepages(zone, -MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
 	__mod_zone_page_state(zone, NR_UNACCEPTED, -MAX_ORDER_NR_PAGES);
 	__ClearPageUnaccepted(page);
···
 	accept_memory(page_to_phys(page), PAGE_SIZE << MAX_PAGE_ORDER);
 
 	__free_pages_ok(page, MAX_PAGE_ORDER, FPI_TO_TAIL);
-
-	if (last) {
-		/*
-		 * There are two corner cases:
-		 *
-		 * - If allocation occurs during the CPU bring up,
-		 *   static_branch_dec() cannot be used directly as
-		 *   it causes a deadlock on cpu_hotplug_lock.
-		 *
-		 *   Instead, use schedule_work() to prevent deadlock.
-		 *
-		 * - If allocation occurs before workqueues are initialized,
-		 *   static_branch_dec() should be called directly.
-		 *
-		 *   Workqueues are initialized before CPU bring up, so this
-		 *   will not conflict with the first scenario.
-		 */
-		if (system_wq)
-			schedule_work(&zone->unaccepted_cleanup);
-		else
-			unaccepted_cleanup_work(&zone->unaccepted_cleanup);
-	}
 }
 
 void accept_page(struct page *page)
···
 	return true;
 }
 
-static inline bool has_unaccepted_memory(void)
-{
-	return static_branch_unlikely(&zones_with_unaccepted_pages);
-}
-
 static bool cond_accept_memory(struct zone *zone, unsigned int order,
 			       int alloc_flags)
 {
 	long to_accept, wmark;
 	bool ret = false;
-
-	if (!has_unaccepted_memory())
-		return false;
 
 	if (list_empty(&zone->unaccepted_pages))
 		return false;
···
 {
 	struct zone *zone = page_zone(page);
 	unsigned long flags;
-	bool first = false;
 
 	if (!lazy_accept)
 		return false;
 
 	spin_lock_irqsave(&zone->lock, flags);
-	first = list_empty(&zone->unaccepted_pages);
 	list_add_tail(&page->lru, &zone->unaccepted_pages);
 	account_freepages(zone, MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
 	__mod_zone_page_state(zone, NR_UNACCEPTED, MAX_ORDER_NR_PAGES);
 	__SetPageUnaccepted(page);
 	spin_unlock_irqrestore(&zone->lock, flags);
-
-	if (first)
-		static_branch_inc(&zones_with_unaccepted_pages);
 
 	return true;
 }