From: Hui Zhu 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"), and say at each site why both stale directions are harmless. 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 Reviewed-by: Dev Jain Acked-by: Zi Yan --- Changelog: v3: Reword both comments to explain why a stale read is harmless (Vlastimil). v2: Real name in Signed-off-by, add Fixes: and Cc: stable+noautosel tags. mm/compaction.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/compaction.c b/mm/compaction.c index f1b2060eb2016..426563acf7acf 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1601,7 +1601,12 @@ static void fast_isolate_freepages(struct compact_control *cc) unsigned int order_scanned = 0; unsigned long high_pfn = 0; - if (!area->nr_free) + /* + * Opportunistic lockless check. A false non-zero is fine, we + * inspect the freelist under the lock. A false zero is fine, + * we just miss this order. + */ + if (!data_race(area->nr_free)) continue; spin_lock_irqsave(&cc->zone->lock, flags); @@ -2031,7 +2036,12 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) unsigned long flags; struct page *freepage; - if (!area->nr_free) + /* + * Opportunistic lockless check. A false non-zero is fine, we + * inspect the freelist under the lock. A false zero is fine, + * we just miss this order. + */ + if (!data_race(area->nr_free)) continue; spin_lock_irqsave(&cc->zone->lock, flags); -- 2.43.0