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BUG: KCSAN: data-race in ext4_write_end / mpage_submit_page

write to 0xffff88821ab9eb38 of 8 bytes by task 20478 on cpu 0:
 i_size_write include/linux/fs.h:896 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3109 [inline]
 ext4_write_end+0x1c1/0x9d0 fs/ext4/inode.c:1294
 generic_perform_write+0x233/0x380 mm/filemap.c:3310
 ext4_buffered_write_iter+0x2b1/0x390 fs/ext4/file.c:270
 ext4_file_write_iter+0x4a3/0xfd0 fs/ext4/file.c:498
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88821ab9eb38 of 8 bytes by task 14099 on cpu 1:
 i_size_read include/linux/fs.h:874 [inline]
 mpage_submit_page+0x67/0x170 fs/ext4/inode.c:2076
 mpage_process_page_bufs+0x38a/0x420 fs/ext4/inode.c:2196
 mpage_prepare_extent_to_map+0x497/0x600 fs/ext4/inode.c:2608
 ext4_writepages+0x7e2/0x1cb0 fs/ext4/inode.c:2736
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __writeback_single_inode+0x81/0x610 fs/fs-writeback.c:1452
 writeback_sb_inodes+0x5b8/0xe00 fs/fs-writeback.c:1716
 wb_writeback+0x2b8/0x730 fs/fs-writeback.c:1892
 wb_do_writeback+0x108/0x600 fs/fs-writeback.c:2037
 wb_workfn+0xb8/0x430 fs/fs-writeback.c:2078
 process_one_work+0x3cb/0x990 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x210/0x220 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 14099 Comm: kworker/u4:7 Not tainted 5.8.0-rc2-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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