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.int... 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: 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 (cherry picked from commit fefc075182275057ce607effaa3daa9e6e3bdc73) Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com --- mm/page_alloc.c | 23 ----------------------- 1 file changed, 23 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index d29da0c6a7f2..ebe1ec661492 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -7041,9 +7041,6 @@ 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) @@ -7070,11 +7067,7 @@ 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); @@ -7083,9 +7076,6 @@ 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) - static_branch_dec(&zones_with_unaccepted_pages); }
void accept_page(struct page *page) @@ -7122,19 +7112,11 @@ 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) { long to_accept, wmark; bool ret = false;
- if (!has_unaccepted_memory()) - return false; - if (list_empty(&zone->unaccepted_pages)) return false;
@@ -7168,22 +7150,17 @@ 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; }