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Subject[PATCH -next v2] mm: annotate a data race in page_zonenum()
Date
 BUG: KCSAN: data-race in page_cpupid_xchg_last / put_page

write (marked) to 0xfffffc0d48ec1a00 of 8 bytes by task 91442 on cpu 3:
page_cpupid_xchg_last+0x51/0x80
page_cpupid_xchg_last at mm/mmzone.c:109 (discriminator 11)
wp_page_reuse+0x3e/0xc0
wp_page_reuse at mm/memory.c:2453
do_wp_page+0x472/0x7b0
do_wp_page at mm/memory.c:2798
__handle_mm_fault+0xcb0/0xd00
handle_pte_fault at mm/memory.c:4049
(inlined by) __handle_mm_fault at mm/memory.c:4163
handle_mm_fault+0xfc/0x2f0
handle_mm_fault at mm/memory.c:4200
do_page_fault+0x263/0x6f9
do_user_addr_fault at arch/x86/mm/fault.c:1465
(inlined by) do_page_fault at arch/x86/mm/fault.c:1539
page_fault+0x34/0x40

read to 0xfffffc0d48ec1a00 of 8 bytes by task 94817 on cpu 69:
put_page+0x15a/0x1f0
page_zonenum at include/linux/mm.h:923
(inlined by) is_zone_device_page at include/linux/mm.h:929
(inlined by) page_is_devmap_managed at include/linux/mm.h:948
(inlined by) put_page at include/linux/mm.h:1023
wp_page_copy+0x571/0x930
wp_page_copy at mm/memory.c:2615
do_wp_page+0x107/0x7b0
__handle_mm_fault+0xcb0/0xd00
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40

Reported by Kernel Concurrency Sanitizer on:
CPU: 69 PID: 94817 Comm: systemd-udevd Tainted: G W O L 5.5.0-next-20200204+ #6
Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019

A page never changes its zone number. The zone number happens to be
stored in the same word as other bits which are modified, but the zone
number bits will never be modified by any other write, so it can accept
a reload of the zone bits after an intervening write and it don't need
to use READ_ONCE(). Thus, annotate this data race using
ASSERT_EXCLUSIVE_BITS() to also assert that there are no concurrent
writes to it.

Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: use ASSERT_EXCLUSIVE_BITS().

BTW, not sure if it is easier for Andrew with Paul to pick this up (with
Andrew's ACK), since ASSERT_EXCLUSIVE_BITS() is in -rcu tree only (or likely
tomorrow's -next tree).

include/linux/mm.h | 1 +
1 file changed, 1 insertion(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 52269e56c514..0d70fafd055c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -920,6 +920,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,

static inline enum zone_type page_zonenum(const struct page *page)
{
+ ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
}

--
1.8.3.1
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