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BUG: KCSAN: data-race in ext4_setattr / mpage_process_page_bufs

write to 0xffff8880ba9682f8 of 8 bytes by task 24460 on cpu 0:
 i_size_write include/linux/fs.h:896 [inline]
 ext4_setattr+0xd50/0xeb0 fs/ext4/inode.c:5386
 notify_change+0x7d4/0xa20 fs/attr.c:336
 do_truncate+0xe0/0x120 fs/open.c:64
 handle_truncate fs/namei.c:2901 [inline]
 do_open fs/namei.c:3247 [inline]
 path_openat+0x1a4c/0x20e0 fs/namei.c:3360
 do_filp_open+0xbd/0x1d0 fs/namei.c:3387
 do_sys_openat2+0x33c/0x4a0 fs/open.c:1179
 do_sys_open fs/open.c:1195 [inline]
 ksys_open include/linux/syscalls.h:1388 [inline]
 __do_sys_creat fs/open.c:1263 [inline]
 __se_sys_creat fs/open.c:1261 [inline]
 __x64_sys_creat+0x62/0x80 fs/open.c:1261
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880ba9682f8 of 8 bytes by task 24455 on cpu 1:
 i_size_read include/linux/fs.h:874 [inline]
 mpage_process_page_bufs+0x36/0x420 fs/ext4/inode.c:2174
 mpage_prepare_extent_to_map+0x497/0x600 fs/ext4/inode.c:2608
 ext4_writepages+0xab7/0x1cb0 fs/ext4/inode.c:2780
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:421
 __filemap_fdatawrite mm/filemap.c:429 [inline]
 filemap_flush+0x1f/0x30 mm/filemap.c:456
 ext4_alloc_da_blocks+0x4c/0x130 fs/ext4/inode.c:3146
 ext4_release_file+0x36/0x180 fs/ext4/file.c:142
 __fput+0x1df/0x460 fs/file_table.c:281
 ____fput+0x11/0x20 fs/file_table.c:314
 task_work_run+0x8e/0x110 kernel/task_work.c:123
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop arch/x86/entry/common.c:216 [inline]
 __prepare_exit_to_usermode+0x1c8/0x230 arch/x86/entry/common.c:246
 __syscall_return_slowpath arch/x86/entry/common.c:329 [inline]
 do_syscall_64+0xb9/0x120 arch/x86/entry/common.c:368
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24455 Comm: syz-executor.0 Not tainted 5.8.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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