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BUG: KCSAN: data-race in shmem_getpage_gfp / shmem_write_end

write to 0xffff888097dbe408 of 8 bytes by task 19100 on cpu 1:
 i_size_write include/linux/fs.h:888 [inline]
 shmem_write_end+0x83/0x4f0 mm/shmem.c:2498
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3320
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3438
 generic_file_write_iter+0x28c/0x38c mm/filemap.c:3470
 call_write_iter include/linux/fs.h:1902 [inline]
 do_iter_readv_writev+0x487/0x5b0 fs/read_write.c:693
 do_iter_write fs/read_write.c:970 [inline]
 do_iter_write+0x13b/0x3c0 fs/read_write.c:951
 vfs_writev+0x118/0x1c0 fs/read_write.c:1015
 do_pwritev+0x131/0x1d0 fs/read_write.c:1112
 __do_sys_pwritev fs/read_write.c:1159 [inline]
 __se_sys_pwritev fs/read_write.c:1154 [inline]
 __x64_sys_pwritev+0x61/0x80 fs/read_write.c:1154
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888097dbe408 of 8 bytes by task 19121 on cpu 0:
 shmem_getpage_gfp+0x792/0x1390 mm/shmem.c:1764
 shmem_getpage mm/shmem.c:154 [inline]
 shmem_file_read_iter+0x224/0x720 mm/shmem.c:2564
 call_read_iter include/linux/fs.h:1896 [inline]
 generic_file_splice_read+0x35c/0x500 fs/splice.c:313
 do_splice_to+0xf2/0x130 fs/splice.c:892
 splice_direct_to_actor+0x1b6/0x540 fs/splice.c:971
 do_splice_direct+0x161/0x1e0 fs/splice.c:1080
 do_sendfile+0x384/0x7f0 fs/read_write.c:1464
 __do_sys_sendfile64 fs/read_write.c:1525 [inline]
 __se_sys_sendfile64 fs/read_write.c:1511 [inline]
 __x64_sys_sendfile64+0x12a/0x140 fs/read_write.c:1511
 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: 19121 Comm: syz-executor.1 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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