On 2/5/20 7:52 PM, Qian Cai wrote:
The commit 07d802699528 ("mm: devmap: refactor 1-based refcounting for
ZONE_DEVICE pages") introduced a data race as page->flags could be
Hi, I really don't think so. This "race" was there long before that commit. Anyway, more below:
accessed concurrently as noticied by KCSAN, 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 Both the read and write are done only with the non-exclusive mmap_sem held. Since the read only check for a specific bit in the flag, even if
Perhaps a clearer explanation is that the read of the page flags is always looking at a bit range (zone number: up to 3 bits) that is not being written to by the writer.
load tearing happens, it will be harmless, so just mark it as an intentional data races using the data_race() macro. Signed-off-by: Qian Cai <cai@xxxxxx> --- include/linux/mm.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 52269e56c514..cafccad584c2 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -920,7 +920,7 @@ vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf);static inline enum zone_type page_zonenum(const struct page *page){ - return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK; + return data_race((page->flags >> ZONES_PGSHIFT) & ZONES_MASK);
I don't know about this. Lots of the kernel is written to do this sort of thing, and adding a load of "data_race()" everywhere is...well, I'm not sure if it's really the best way. I wonder: could we maybe teach this kcsan thing to understand a few of the key idioms, particularly about page flags, instead of annotating all over the place? thanks, -- John Hubbard NVIDIA
}#ifdef CONFIG_ZONE_DEVICE
