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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_mfill_atomic_pte

write to 0xffff8881217fd658 of 8 bytes by task 10012 on cpu 1:
 shmem_add_to_page_cache+0x5e7/0x910 mm/shmem.c:656
 shmem_getpage_gfp+0x831/0x1940 mm/shmem.c:1857
 shmem_fault+0x16c/0x430 mm/shmem.c:2044
 __do_fault mm/memory.c:3464 [inline]
 do_read_fault+0x41f/0x730 mm/memory.c:3857
 do_fault mm/memory.c:3985 [inline]
 handle_pte_fault mm/memory.c:4225 [inline]
 __handle_mm_fault mm/memory.c:4357 [inline]
 handle_mm_fault+0x135d/0x1930 mm/memory.c:4394
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:565
 fault_in_pages_readable arch/x86/include/asm/smap.h:53 [inline]
 iov_iter_fault_in_readable+0x14f/0x390 lib/iov_iter.c:431
 generic_perform_write+0xd2/0x390 mm/filemap.c:3289
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:270
 ext4_file_write_iter+0x76f/0x1010 include/linux/fs.h:804
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881217fd658 of 8 bytes by task 10002 on cpu 0:
 shmem_recalc_inode mm/shmem.c:305 [inline]
 shmem_mfill_atomic_pte+0x992/0xf70 mm/shmem.c:2383
 shmem_mcopy_atomic_pte+0x3a/0x50 mm/shmem.c:2416
 mfill_atomic_pte mm/userfaultfd.c:446 [inline]
 __mcopy_atomic mm/userfaultfd.c:579 [inline]
 mcopy_atomic+0x620/0x13b0 mm/userfaultfd.c:629
 userfaultfd_copy fs/userfaultfd.c:1743 [inline]
 userfaultfd_ioctl+0x1126/0x2530 fs/userfaultfd.c:1941
 vfs_ioctl fs/ioctl.c:48 [inline]
 ksys_ioctl fs/ioctl.c:753 [inline]
 __do_sys_ioctl fs/ioctl.c:762 [inline]
 __se_sys_ioctl+0xc9/0x130 fs/ioctl.c:760
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:760
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10002 Comm: syz-executor.0 Not tainted 5.8.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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