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

[Dev Jain]

On 28/10/26 2:37 pm, Hui Zhu wrote:
> From: teawater <zhuhui@xxxxxxxxxx>
> 
> 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.
> 
> Signed-off-by: teawater <zhuhui@xxxxxxxxxx>

teawater? What?


> ---
>  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.
> +		 */

The comment is noise, the reason of using data_race() is a well-known fact.

Apart from that, do we have the same problem with zone_watermark_ok? It also
uses area->nr_free without zone lock.

> +		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);