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BUG: KCSAN: data-race in ext4_setattr / mpage_process_page_bufs

write to 0xffff88811eb18338 of 8 bytes by task 19548 on cpu 1:
 i_size_write include/linux/fs.h:876 [inline]
 ext4_setattr+0xd63/0xec0 fs/ext4/inode.c:5386
 notify_change+0x7d7/0xa80 fs/attr.c:336
 do_truncate+0xe0/0x120 fs/open.c:64
 handle_truncate fs/namei.c:2909 [inline]
 do_open fs/namei.c:3255 [inline]
 path_openat+0x1a31/0x20a0 fs/namei.c:3368
 do_filp_open+0xbd/0x1d0 fs/namei.c:3395
 do_sys_openat2+0x33b/0x500 fs/open.c:1168
 do_sys_open fs/open.c:1184 [inline]
 __do_sys_creat fs/open.c:1258 [inline]
 __se_sys_creat fs/open.c:1252 [inline]
 __x64_sys_creat+0x62/0x80 fs/open.c:1252
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811eb18338 of 8 bytes by task 19542 on cpu 0:
 i_size_read include/linux/fs.h:854 [inline]
 mpage_process_page_bufs+0x34/0x600 fs/ext4/inode.c:2173
 mpage_prepare_extent_to_map+0x4ec/0x650 fs/ext4/inode.c:2607
 ext4_writepages+0x97d/0x1ef0 fs/ext4/inode.c:2779
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:422
 file_write_and_wait_range+0x9f/0x120 mm/filemap.c:761
 ext4_sync_file+0xfc/0x6e0 fs/ext4/fsync.c:151
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2747 [inline]
 ext4_buffered_write_iter+0x369/0x3b0 fs/ext4/file.c:276
 ext4_file_write_iter+0x768/0x1060 include/linux/fs.h:784
 call_write_iter include/linux/fs.h:1882 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x665/0x6f0 fs/read_write.c:578
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 19542 Comm: syz-executor.1 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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