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.

Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Fixes: dcdfdd40fa82 ("mm: Add support for unaccepted memory") Link: https://lore.kernel.org/all/20250506092445.GBaBnVXXyvnazly6iF@fat_crate.loca... Reported-by: Borislav Petkov bp@alien8.de Tested-by: Borislav Petkov (AMD) bp@alien8.de Reported-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org # v6.5+ Cc: Andrew Morton akpm@linux-foundation.org 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 ---

v2: - Update commit message; - Apply Borislav's Tested-by tag;

--- mm/internal.h | 1 - mm/mm_init.c | 1 - mm/page_alloc.c | 47 ----------------------------------------------- 3 files changed, 49 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h index e9695baa5922..50c2f590b2d0 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -1595,7 +1595,6 @@ unsigned long move_page_tables(struct pagetable_move_control *pmc);

#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) { diff --git a/mm/mm_init.c b/mm/mm_init.c index 9659689b8ace..84f14fa12d0d 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -1441,7 +1441,6 @@ static void __meminit zone_init_free_lists(struct zone *zone)

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

diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 5fccf5fce084..a4a4df2daedb 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -7175,16 +7175,8 @@ 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;

-void unaccepted_cleanup_work(struct work_struct *work) -{ - static_branch_dec(&zones_with_unaccepted_pages); -} - static int __init accept_memory_parse(char *p) { if (!strcmp(p, "lazy")) { @@ -7209,11 +7201,7 @@ static bool page_contains_unaccepted(struct page *page, unsigned int order) 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); @@ -7222,28 +7210,6 @@ static void __accept_page(struct zone *zone, unsigned long *flags, 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) @@ -7280,20 +7246,12 @@ static bool try_to_accept_memory_one(struct zone *zone) 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;

@@ -7331,22 +7289,17 @@ 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); 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; }