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BUG: KCSAN: data-race in filemap_map_pages / shmem_write_end

write to 0xffff88812065a140 of 8 bytes by task 14064 on cpu 1:
 i_size_write include/linux/fs.h:888 [inline]
 shmem_write_end+0x83/0x4f0 mm/shmem.c:2499
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3298
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3416
 generic_file_write_iter+0x28c/0x38c mm/filemap.c:3448
 call_write_iter include/linux/fs.h:1901 [inline]
 do_iter_readv_writev+0x487/0x5b0 fs/read_write.c:693
 do_iter_write fs/read_write.c:998 [inline]
 do_iter_write+0x13b/0x3c0 fs/read_write.c:979
 vfs_iter_write+0x5c/0x80 fs/read_write.c:1039
 iter_file_splice_write+0x536/0x850 fs/splice.c:760
 do_splice_from fs/splice.c:863 [inline]
 direct_splice_actor+0xa0/0xc0 fs/splice.c:1037
 splice_direct_to_actor+0x22b/0x540 fs/splice.c:992
 do_splice_direct+0x161/0x1e0 fs/splice.c:1080
 do_sendfile+0x384/0x7f0 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1575 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0xbe/0x140 fs/read_write.c:1567
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812065a140 of 8 bytes by task 14066 on cpu 0:
 filemap_map_pages+0x812/0x990 mm/filemap.c:2621
 do_fault_around mm/memory.c:3634 [inline]
 do_read_fault mm/memory.c:3668 [inline]
 do_fault mm/memory.c:3802 [inline]
 handle_pte_fault mm/memory.c:4033 [inline]
 __handle_mm_fault+0x2a92/0x2e00 mm/memory.c:4163
 handle_mm_fault+0x21b/0x530 mm/memory.c:4200
 do_user_addr_fault arch/x86/mm/fault.c:1442 [inline]
 do_page_fault+0x496/0xa3d arch/x86/mm/fault.c:1517
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203
 do_strnlen_user lib/strnlen_user.c:35 [inline]
 strnlen_user+0xec/0x220 lib/strnlen_user.c:113
 __do_sys_memfd_create mm/memfd.c:271 [inline]
 __se_sys_memfd_create mm/memfd.c:250 [inline]
 __x64_sys_memfd_create+0x93/0x370 mm/memfd.c:250
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14066 Comm: syz-executor.0 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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