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BUG: KCSAN: data-race in filemap_map_pages / shmem_write_end

write to 0xffff8880b0b0b490 of 8 bytes by task 13553 on cpu 1:
 i_size_write include/linux/fs.h:896 [inline]
 shmem_write_end+0x62/0x4e0 mm/shmem.c:2470
 generic_perform_write+0x23b/0x390 mm/filemap.c:3310
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3428
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3460
 call_write_iter include/linux/fs.h:1907 [inline]
 do_iter_readv_writev+0x321/0x3c0 fs/read_write.c:694
 do_iter_write+0x10e/0x470 fs/read_write.c:999
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1040
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x562/0xb10 fs/read_write.c:1521
 __do_sys_sendfile64 fs/read_write.c:1576 [inline]
 __se_sys_sendfile64 fs/read_write.c:1568 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1568
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b0b0b490 of 8 bytes by task 13556 on cpu 0:
 i_size_read include/linux/fs.h:874 [inline]
 filemap_map_pages+0x654/0x990 mm/filemap.c:2625
 do_fault_around mm/memory.c:3818 [inline]
 do_read_fault+0x22f/0x730 mm/memory.c:3852
 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
 copy_user_enhanced_fast_string+0xe/0x30 arch/x86/lib/copy_user_64.S:204
 copy_user_generic arch/x86/include/asm/uaccess_64.h:37 [inline]
 raw_copy_from_user arch/x86/include/asm/uaccess_64.h:68 [inline]
 _copy_from_user+0x7d/0xd0 lib/usercopy.c:15
 copy_from_user include/linux/uaccess.h:144 [inline]
 __get_filter+0xc7/0x210 net/core/filter.c:1465
 sk_attach_filter+0x21/0x260 net/core/filter.c:1496
 sock_setsockopt+0x1555/0x1c80 net/core/sock.c:1080
 __sys_setsockopt+0x223/0x390 net/socket.c:2123
 __do_sys_setsockopt net/socket.c:2143 [inline]
 __se_sys_setsockopt net/socket.c:2140 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2140
 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: 13556 Comm: syz-executor.2 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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