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BUG: KCSAN: data-race in ext4_write_end / mpage_process_page_bufs

write to 0xffff88810660b290 of 8 bytes by task 6007 on cpu 1:
 i_size_write include/linux/fs.h:871 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3349 [inline]
 ext4_write_end+0x1d7/0x820 fs/ext4/inode.c:1309
 ext4_da_write_end+0x93/0x730 fs/ext4/inode.c:3079
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3671
 ext4_buffered_write_iter+0x2f2/0x3f0 fs/ext4/file.c:269
 ext4_file_write_iter+0x2e7/0x11d0 fs/ext4/file.c:519
 call_write_iter include/linux/fs.h:2114 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x69d/0x770 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88810660b290 of 8 bytes by task 27 on cpu 0:
 i_size_read include/linux/fs.h:849 [inline]
 mpage_process_page_bufs+0x34/0x560 fs/ext4/inode.c:2192
 mpage_prepare_extent_to_map+0x4e6/0x640 fs/ext4/inode.c:2626
 ext4_writepages+0x9d8/0x1d10 fs/ext4/inode.c:2798
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x80/0x450 fs/fs-writeback.c:1467
 writeback_sb_inodes+0x564/0xf10 fs/fs-writeback.c:1732
 __writeback_inodes_wb fs/fs-writeback.c:1801 [inline]
 wb_writeback+0x327/0x700 fs/fs-writeback.c:1907
 wb_check_start_all fs/fs-writeback.c:2031 [inline]
 wb_do_writeback+0x330/0x510 fs/fs-writeback.c:2057
 wb_workfn+0xc8/0x520 fs/fs-writeback.c:2091
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

value changed: 0x0000000000103000 -> 0x0000000000104000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 27 Comm: kworker/u4:2 Not tainted 5.13.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-7:4)
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