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BUG: KCSAN: data-race in next_uptodate_page / shmem_write_end

write to 0xffff88800b47f140 of 8 bytes by task 24146 on cpu 0:
 i_size_write include/linux/fs.h:872 [inline]
 shmem_write_end+0x62/0x650 mm/shmem.c:2541
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3586
 __generic_file_write_iter+0x15c/0x340 mm/filemap.c:3704
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3736
 call_write_iter include/linux/fs.h:1977 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x7a6/0xe20 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1319 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1311
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88800b47f140 of 8 bytes by task 24151 on cpu 1:
 i_size_read include/linux/fs.h:850 [inline]
 next_uptodate_page+0x373/0x780 mm/filemap.c:3061
 first_map_page mm/filemap.c:3078 [inline]
 filemap_map_pages+0xe5/0x770 mm/filemap.c:3104
 do_fault_around mm/memory.c:3920 [inline]
 do_read_fault+0x217/0x530 mm/memory.c:3934
 do_fault mm/memory.c:4067 [inline]
 handle_pte_fault mm/memory.c:4318 [inline]
 __handle_mm_fault mm/memory.c:4453 [inline]
 handle_mm_fault+0x1019/0x1840 mm/memory.c:4551
 do_user_addr_fault+0x60c/0xc00 arch/x86/mm/fault.c:1390
 handle_page_fault arch/x86/mm/fault.c:1475 [inline]
 exc_page_fault+0x94/0x2a0 arch/x86/mm/fault.c:1531
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:577
 copy_user_enhanced_fast_string+0xe/0x40 arch/x86/lib/copy_user_64.S:205
 copy_user_generic arch/x86/include/asm/uaccess_64.h:37 [inline]
 raw_copy_from_user arch/x86/include/asm/uaccess_64.h:52 [inline]
 _copy_from_user+0x7e/0xd0 lib/usercopy.c:16
 copy_from_user include/linux/uaccess.h:192 [inline]
 prctl_set_mm_map+0xb8/0x470 kernel/sys.c:1982
 prctl_set_mm+0x13d/0x8b0 kernel/sys.c:2122
 __do_sys_prctl+0x10a/0xc50 kernel/sys.c:2427
 __se_sys_prctl kernel/sys.c:2283 [inline]
 __x64_sys_prctl+0x62/0x70 kernel/sys.c:2283
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24151 Comm: syz-executor.2 Tainted: G        W         5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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