mm/page_alloc: fix race condition in unaccepted memory handling

commit fefc075 upstream.

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/20250506133207.1009676-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: dcdfdd4 ("mm: Add support for unaccepted memory")
Link: https://lore.kernel.org/all/20250506092445.GBaBnVXXyvnazly6iF@fat_crate.local
Reported-by: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>	[6.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/page_alloc.c

@@ -303,7 +303,6 @@ EXPORT_SYMBOL(nr_online_nodes);
 static bool page_contains_unaccepted(struct page *page, unsigned int order);
 static void accept_page(struct page *page, unsigned int order);
 static bool cond_accept_memory(struct zone *zone, unsigned int order);
-static inline bool has_unaccepted_memory(void);
 static bool __free_unaccepted(struct page *page);
 
 int page_group_by_mobility_disabled __read_mostly;
@@ -6586,9 +6585,6 @@ bool has_managed_dma(void)
 
 #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;
 
 static int __init accept_memory_parse(char *p)
@@ -6624,7 +6620,6 @@ static bool try_to_accept_memory_one(struct zone *zone)
 {
 	unsigned long flags;
 	struct page *page;
-	bool last;
 
 	spin_lock_irqsave(&zone->lock, flags);
 	page = list_first_entry_or_null(&zone->unaccepted_pages,
@@ -6635,7 +6630,6 @@ static bool try_to_accept_memory_one(struct zone *zone)
 	}
 
 	list_del(&page->lru);
-	last = list_empty(&zone->unaccepted_pages);
 
 	__mod_zone_freepage_state(zone, -MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
 	__mod_zone_page_state(zone, NR_UNACCEPTED, -MAX_ORDER_NR_PAGES);
@@ -6645,9 +6639,6 @@ static bool try_to_accept_memory_one(struct zone *zone)
 
 	__free_pages_ok(page, MAX_ORDER, FPI_TO_TAIL);
 
-	if (last)
-		static_branch_dec(&zones_with_unaccepted_pages);
-
 	return true;
 }
 
@@ -6656,9 +6647,6 @@ static bool cond_accept_memory(struct zone *zone, unsigned int order)
 	long to_accept, wmark;
 	bool ret = false;
 
-	if (!has_unaccepted_memory())
-		return false;
-
 	if (list_empty(&zone->unaccepted_pages))
 		return false;
 
@@ -6688,30 +6676,20 @@ static bool cond_accept_memory(struct zone *zone, unsigned int order)
 	return ret;
 }
 
-static inline bool has_unaccepted_memory(void)
-{
-	return static_branch_unlikely(&zones_with_unaccepted_pages);
-}
-
 static bool __free_unaccepted(struct page *page)
 {
 	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);
 	__mod_zone_freepage_state(zone, MAX_ORDER_NR_PAGES, MIGRATE_MOVABLE);
 	__mod_zone_page_state(zone, NR_UNACCEPTED, MAX_ORDER_NR_PAGES);
 	spin_unlock_irqrestore(&zone->lock, flags);
 
-	if (first)
-		static_branch_inc(&zones_with_unaccepted_pages);
-
 	return true;
 }
 
@@ -6731,11 +6709,6 @@ static bool cond_accept_memory(struct zone *zone, unsigned int order)
 	return false;
 }
 
-static inline bool has_unaccepted_memory(void)
-{
-	return false;
-}
-
 static bool __free_unaccepted(struct page *page)
 {
 	BUILD_BUG();