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BUG: KCSAN: data-race in ext4_write_end / ext4_writepages

write to 0xffff8881006afb08 of 8 bytes by task 18275 on cpu 0:
 i_size_write include/linux/fs.h:868 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3141 [inline]
 ext4_write_end+0x1da/0x9c0 fs/ext4/inode.c:1293
 ext4_da_write_end+0x91/0x750 fs/ext4/inode.c:3060
 generic_perform_write+0x23b/0x390 mm/filemap.c:3516
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:269
 ext4_file_write_iter+0x768/0x1060 include/linux/fs.h:776
 call_write_iter include/linux/fs.h:1874 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x665/0x6f0 fs/read_write.c:586
 ksys_write+0xce/0x180 fs/read_write.c:639
 __do_sys_write fs/read_write.c:651 [inline]
 __se_sys_write fs/read_write.c:648 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:648
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881006afb08 of 8 bytes by task 7 on cpu 1:
 i_size_read include/linux/fs.h:846 [inline]
 mpage_submit_page fs/ext4/inode.c:2075 [inline]
 mpage_map_and_submit_buffers fs/ext4/inode.c:2329 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2468 [inline]
 ext4_writepages+0xdfe/0x1e40 fs/ext4/inode.c:2781
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x580 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6c0/0xfd0 fs/fs-writeback.c:1721
 wb_writeback+0x28d/0x670 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2039
 wb_workfn+0xc8/0x430 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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