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BUG: KCSAN: data-race in delete_from_page_cache_batch / ext4_writepages

write to 0xffff8880bad622c0 of 8 bytes by task 14029 on cpu 1:
 page_cache_delete_batch mm/filemap.c:340 [inline]
 delete_from_page_cache_batch+0x587/0x800 mm/filemap.c:358
 truncate_inode_pages_range+0x482/0xf60 mm/truncate.c:366
 truncate_inode_pages mm/truncate.c:476 [inline]
 truncate_pagecache+0x4f/0x70 mm/truncate.c:839
 ext4_setattr+0xaea/0xe90 fs/ext4/inode.c:5403
 notify_change+0x7d7/0xa80 fs/attr.c:336
 do_truncate fs/open.c:64 [inline]
 do_sys_ftruncate+0x39f/0x3e0 fs/open.c:195
 __do_sys_ftruncate fs/open.c:205 [inline]
 __se_sys_ftruncate fs/open.c:203 [inline]
 __x64_sys_ftruncate+0x2f/0x40 fs/open.c:203
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bad622c0 of 8 bytes by task 21 on cpu 0:
 ext4_writepages+0x133/0x1e40 fs/ext4/inode.c:2649
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x580 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6c0/0xfd0 fs/fs-writeback.c:1721
 wb_writeback+0x28d/0x670 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2039
 wb_workfn+0xc8/0x430 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 21 Comm: kworker/u4:1 Not tainted 5.9.0-syzkaller #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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