* [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free
@ 2026-10-28 9:07 Hui Zhu
2026-10-28 10:30 ` Dev Jain
0 siblings, 1 reply; 3+ messages in thread
From: Hui Zhu @ 2026-10-28 9:07 UTC (permalink / raw)
To: Andrew Morton, Vlastimil Babka, Suren Baghdasaryan, Michal Hocko,
Brendan Jackman, Johannes Weiner, Zi Yan, linux-mm, linux-kernel
Cc: teawater
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>
---
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
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free
2026-10-28 9:07 [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free Hui Zhu
@ 2026-10-28 10:30 ` Dev Jain
2026-10-28 13:46 ` Zi Yan
0 siblings, 1 reply; 3+ messages in thread
From: Dev Jain @ 2026-10-28 10:30 UTC (permalink / raw)
To: Hui Zhu, Andrew Morton, Vlastimil Babka, Suren Baghdasaryan,
Michal Hocko, Brendan Jackman, Johannes Weiner, Zi Yan, linux-mm,
linux-kernel
Cc: teawater
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.
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);
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free
2026-10-28 10:30 ` Dev Jain
@ 2026-10-28 13:46 ` Zi Yan
0 siblings, 0 replies; 3+ messages in thread
From: Zi Yan @ 2026-10-28 13:46 UTC (permalink / raw)
To: Dev Jain, Hui Zhu, Andrew Morton, Vlastimil Babka,
Suren Baghdasaryan, Michal Hocko, Brendan Jackman,
Johannes Weiner, linux-mm, linux-kernel
Cc: teawater
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.
>
> 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);
--
Best Regards,
Yan, Zi
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-10-28 9:07 [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free Hui Zhu
2026-10-28 10:30 ` Dev Jain
2026-10-28 13:46 ` Zi Yan
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