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BUG: KCSAN: data-race in generic_write_end / mpage_process_page_bufs

write to 0xffff88811cc91718 of 8 bytes by task 3376 on cpu 0:
 i_size_write include/linux/fs.h:907 [inline]
 generic_write_end+0x96/0x180 fs/buffer.c:2187
 ext4_da_write_end+0x4de/0x630 fs/ext4/inode.c:3042
 generic_perform_write+0x261/0x400 mm/filemap.c:3776
 ext4_buffered_write_iter+0x244/0x360 fs/ext4/file.c:268
 ext4_file_write_iter+0x2e7/0x11f0
 call_write_iter include/linux/fs.h:2074 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x7f5/0x950 fs/read_write.c:590
 ksys_write+0xd9/0x190 fs/read_write.c:643
 __do_sys_write fs/read_write.c:655 [inline]
 __se_sys_write fs/read_write.c:652 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:652
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88811cc91718 of 8 bytes by task 230 on cpu 1:
 i_size_read include/linux/fs.h:885 [inline]
 mpage_process_page_bufs+0x34/0x560 fs/ext4/inode.c:2172
 mpage_prepare_extent_to_map+0x4f6/0x650 fs/ext4/inode.c:2604
 ext4_writepages+0xa78/0x1e80 fs/ext4/inode.c:2776
 do_writepages+0x1cb/0x330 mm/page-writeback.c:2364
 __writeback_single_inode+0x81/0x590 fs/fs-writeback.c:1618
 writeback_sb_inodes+0x677/0xdf0 fs/fs-writeback.c:1891
 __writeback_inodes_wb+0xb0/0x2b0 fs/fs-writeback.c:1961
 wb_writeback+0x280/0x650 fs/fs-writeback.c:2066
 wb_check_start_all fs/fs-writeback.c:2188 [inline]
 wb_do_writeback+0x378/0x560 fs/fs-writeback.c:2214
 wb_workfn+0xc8/0x440 fs/fs-writeback.c:2248
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x1bf/0x1e0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30

value changed: 0x00000000002c1000 -> 0x00000000002c2000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 230 Comm: kworker/u4:3 Not tainted 5.17.0-rc2-syzkaller-00039-g9f7fb8de5d9b #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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