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BUG: KCSAN: data-race in ext4_setattr / mpage_process_page_bufs

write to 0xffff8880b20e0638 of 8 bytes by task 21645 on cpu 1:
 i_size_write include/linux/fs.h:888 [inline]
 ext4_setattr+0xca8/0x15a0 fs/ext4/inode.c:5642
 notify_change+0x7e1/0xaa0 fs/attr.c:337
 do_truncate+0xfb/0x180 fs/open.c:64
 handle_truncate fs/namei.c:2998 [inline]
 do_last fs/namei.c:3414 [inline]
 path_openat+0x2f48/0x36e0 fs/namei.c:3525
 do_filp_open+0x11e/0x1b0 fs/namei.c:3555
 do_sys_open+0x3b3/0x4f0 fs/open.c:1097
 __do_sys_openat fs/open.c:1124 [inline]
 __se_sys_openat fs/open.c:1118 [inline]
 __x64_sys_openat+0x62/0x80 fs/open.c:1118
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b20e0638 of 8 bytes by task 21642 on cpu 0:
 mpage_process_page_bufs+0x40/0x400 fs/ext4/inode.c:2314
 mpage_prepare_extent_to_map+0x48b/0x760 fs/ext4/inode.c:2694
 ext4_writepages+0xb2b/0x2300 fs/ext4/inode.c:2866
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1c5/0x230 mm/filemap.c:421
 __filemap_fdatawrite mm/filemap.c:429 [inline]
 filemap_flush+0x2d/0x40 mm/filemap.c:456
 ext4_alloc_da_blocks+0x78/0x180 fs/ext4/inode.c:3243
 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+0x2f/0x6c0 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 21642 Comm: syz-executor.1 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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