[PATCH v2] mm/compaction: annotate data race for zone->free_area[order].nr_free

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

KCSAN reports a data-race on nr_free which also exists on mainline:

  BUG: KCSAN: data-race in fast_isolate_freepages+0x284/0x1550 mm/compaction.c:1589

  race at unknown origin, with read to 0xffff9894ff3dcb60 of 8 bytes by task 179078 on cpu 12:
   fast_isolate_freepages+0x284/0x1550 mm/compaction.c:1589
   isolate_freepages+0x52/0x650 mm/compaction.c:1722
   compaction_alloc_noprof+0x36e/0x3e0 mm/compaction.c:1850
   compaction_alloc+0x21/0x30 mm/compaction.c:1880
   migrate_folio_unmap+0x7f/0x820 mm/migrate.c:1177
   migrate_pages_batch+0x37e/0x1200 mm/migrate.c:1762
   migrate_pages_sync+0xce/0x560 mm/migrate.c:1928
   migrate_pages+0x8c6/0xc20 mm/migrate.c:2037
   compact_zone+0x539/0x10b0 mm/compaction.c:2660
   compact_zone_order+0x1b8/0x230 mm/compaction.c:2795
   try_to_compact_pages+0x20b/0x630 mm/compaction.c:2851
   __alloc_pages_direct_compact+0x98/0x2b0 mm/page_alloc.c:3722
   __alloc_pages_slowpath.constprop.0+0x452/0xdf0 mm/page_alloc.c:4401
   __alloc_pages_noprof+0x28a/0x2d0 mm/page_alloc.c:4794
   alloc_pages_mpol_noprof+0x1b1/0x470 mm/mempolicy.c:2270
   alloc_pages_noprof+0x7a/0xe0 mm/mempolicy.c:2347
   io_mem_alloc_compound io_uring/io_uring.c:2848 [inline]
   io_pages_map+0xb9/0x330 io_uring/io_uring.c:2892
   io_allocate_scq_urings+0x231/0x300 io_uring/io_uring.c:3933
   io_uring_create+0x29c/0x9c0 io_uring/io_uring.c:4091
   io_uring_setup+0xb6/0x100 io_uring/io_uring.c:4203
   __do_sys_io_uring_setup io_uring/io_uring.c:4230 [inline]
   __se_sys_io_uring_setup io_uring/io_uring.c:4224 [inline]
   __x64_sys_io_uring_setup+0x66/0xe0 io_uring/io_uring.c:4224
   x64_sys_call+0x1816/0x1d90 arch/x86/include/generated/asm/syscalls_64.h:426
   do_syscall_x64 arch/x86/entry/common.c:51 [inline]
   do_syscall_64+0x70/0x2a0 arch/x86/entry/common.c:81
   entry_SYSCALL_64_after_hwframe+0x76/0xe0

  value changed: 0x00000000000002f4 -> 0x00000000000002f5

nr_free is only updated with zone->lock held, but fast_isolate_freepages()
and fast_find_migrateblock() read it locklessly to skip an order that has
no free pages.  page_alloc.o is excluded from KCSAN, so the writer is not
instrumented and the report is of unknown origin.

The race is benign: a stale zero only skips the order for this round, and
a stale non-zero just finds an empty MIGRATE_MOVABLE freelist under the
lock.  Annotate both reads with data_race(), following commit
af1c31acc853 ("mm/vmstat: annotate data race for zone->free_area[order].nr_free").

No functional change intended.

Fixes: 70b44595eafe ("mm, compaction: use free lists to quickly locate a migration source")
Fixes: 5a811889de10 ("mm, compaction: use free lists to quickly locate a migration target")
Cc: stable+noautosel@kernel.org # KCSAN annotation only, no functional change
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: Zi Yan <ziy@nvidia.com>
---
 mm/compaction.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f1b2060eb2016..499ec7ceaa3ed 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1601,7 +1601,11 @@ static void fast_isolate_freepages(struct compact_control *cc)
 		unsigned int order_scanned = 0;
 		unsigned long high_pfn = 0;
 
-		if (!area->nr_free)
+		/*
+		 * Access to nr_free is lockless as it is only a hint; a stale
+		 * value is harmless. Use data_race to avoid KCSAN warning.
+		 */
+		if (!data_race(area->nr_free))
 			continue;
 
 		spin_lock_irqsave(&cc->zone->lock, flags);
@@ -2031,7 +2035,11 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 		unsigned long flags;
 		struct page *freepage;
 
-		if (!area->nr_free)
+		/*
+		 * Access to nr_free is lockless as it is only a hint; a stale
+		 * value is harmless. Use data_race to avoid KCSAN warning.
+		 */
+		if (!data_race(area->nr_free))
 			continue;
 
 		spin_lock_irqsave(&cc->zone->lock, flags);
-- 
2.43.0