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BUG: KCSAN: data-race in shmem_fallocate / shmem_fault

write to 0xffff88805747fb38 of 8 bytes by task 3669 on cpu 1:
 shmem_fallocate+0x1f1/0x830 mm/shmem.c:2789
 vfs_fallocate+0x43c/0x5d0 fs/open.c:309
 madvise_remove mm/madvise.c:857 [inline]
 madvise_vma mm/madvise.c:940 [inline]
 do_madvise+0x83d/0x14f0 mm/madvise.c:1145
 __do_sys_madvise mm/madvise.c:1171 [inline]
 __se_sys_madvise mm/madvise.c:1169 [inline]
 __x64_sys_madvise+0x3e/0x50 mm/madvise.c:1169
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88805747fb38 of 8 bytes by task 3634 on cpu 0:
 shmem_fault+0x90/0x430 mm/shmem.c:2057
 __do_fault mm/memory.c:3639 [inline]
 do_read_fault+0x41f/0x760 mm/memory.c:4032
 do_fault mm/memory.c:4160 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x14e9/0x1af0 mm/memory.c:4633
 faultin_page mm/gup.c:878 [inline]
 __get_user_pages+0xa3d/0x1010 mm/gup.c:1090
 populate_vma_page_range mm/gup.c:1423 [inline]
 __mm_populate+0x24d/0x340 mm/gup.c:1471
 mm_populate include/linux/mm.h:2566 [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: 0 PID: 3634 Comm: syz-executor.3 Not tainted 5.9.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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