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BUG: KCSAN: data-race in expand_downwards / handle_mm_fault

write to 0xffff888125f083b8 of 8 bytes by task 12032 on cpu 0:
 expand_downwards+0x6f2/0x9c0 mm/mmap.c:2508
 expand_stack+0x19/0x20 mm/mmap.c:2564
 do_user_addr_fault+0x67e/0x770 arch/x86/mm/fault.c:1344
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538
 do_strncpy_from_user lib/strncpy_from_user.c:40 [inline]
 strncpy_from_user+0x103/0x2e0 lib/strncpy_from_user.c:121
 getname_flags+0xb8/0x3d0 fs/namei.c:149
 getname+0x15/0x20 fs/namei.c:209
 do_sys_openat2+0x2f2/0x500 fs/open.c:1162
 do_sys_open fs/open.c:1184 [inline]
 __do_sys_openat fs/open.c:1200 [inline]
 __se_sys_openat fs/open.c:1195 [inline]
 __x64_sys_openat+0xef/0x110 fs/open.c:1195
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888125f083b8 of 8 bytes by task 12024 on cpu 1:
 linear_page_index include/linux/pagemap.h:495 [inline]
 __handle_mm_fault mm/memory.c:4267 [inline]
 handle_mm_fault+0x238/0x1af0 mm/memory.c:4444
 faultin_page mm/gup.c:878 [inline]
 __get_user_pages+0xa3d/0x1010 mm/gup.c:1090
 populate_vma_page_range mm/gup.c:1420 [inline]
 __mm_populate+0x24d/0x340 mm/gup.c:1468
 mm_populate include/linux/mm.h:2564 [inline]
 vm_mmap_pgoff+0x133/0x170 mm/util.c:511
 ksys_mmap_pgoff+0xe1/0x380 mm/mmap.c:1596
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12024 Comm: syz-executor.2 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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