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BUG: KCSAN: data-race in generic_write_end / mpage_process_page_bufs

write to 0xffff888125d34a18 of 8 bytes by task 7747 on cpu 0:
 i_size_write include/linux/fs.h:875 [inline]
 generic_write_end+0xd0/0x1f0 fs/buffer.c:2143
 ext4_da_write_end+0x158/0x620 fs/ext4/inode.c:3217
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3338
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3456
 ext4_file_write_iter+0x1fc/0xa40 fs/ext4/file.c:270
 call_write_iter include/linux/fs.h:1870 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888125d34a18 of 8 bytes by task 7731 on cpu 1:
 mpage_process_page_bufs+0x41/0x3d0 fs/ext4/inode.c:2345
 mpage_prepare_extent_to_map+0x48b/0x760 fs/ext4/inode.c:2722
 ext4_writepages+0xb4f/0x22e0 fs/ext4/inode.c:2894
 do_writepages+0x6b/0x170 mm/page-writeback.c:2342
 __filemap_fdatawrite_range+0x19e/0x200 mm/filemap.c:415
 __filemap_fdatawrite mm/filemap.c:423 [inline]
 filemap_flush+0x2d/0x40 mm/filemap.c:450
 ext4_alloc_da_blocks+0x78/0x180 fs/ext4/inode.c:3289
 ext4_release_file+0x10d/0x200 fs/ext4/file.c:88
 __fput+0x1e1/0x520 fs/file_table.c:280
 ____fput+0x1f/0x30 fs/file_table.c:313
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 get_signal+0x12c3/0x1320 kernel/signal.c:2528
 do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7731 Comm: syz-executor.2 Not tainted 5.3.0+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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