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

[Dev Jain <dev.jain@arm.com>]

On 28/10/26 7:16 pm, Zi Yan wrote:
> On Wed Oct 28, 2026 at 6:30 AM EDT, Dev Jain wrote:
>>
>>
>> On 28/10/26 2:37 pm, Hui Zhu wrote:
>>> From: teawater <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.
>>>
>>> Signed-off-by: teawater <zhuhui@kylinos.cn>
>>
>> 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.
> 
> IIRC, checkpatch.pl requires a comment for each data_race(). The
> comments need to stay until you send a patch to change checkpatch.pl.

Okay makes sense.


> 
>>
>> 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);
> 
> 
> 
>