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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

read-write to 0xffff88810d35c8f8 of 8 bytes by task 24202 on cpu 0:
 __add_to_page_cache_locked+0x39d/0x7b0 mm/filemap.c:888
 add_to_page_cache_lru+0xa0/0x1b0 mm/filemap.c:939
 pagecache_get_page+0x60f/0x980 mm/filemap.c:1839
 grab_cache_page_write_begin+0x3f/0x70 mm/filemap.c:3261
 ext4_da_write_begin+0x34d/0xaf0 fs/ext4/inode.c:2974
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3311
 ext4_buffered_write_iter+0x2e5/0x3d0 fs/ext4/file.c:270
 ext4_file_write_iter+0x45e/0x1090 fs/ext4/file.c:503
 call_write_iter include/linux/fs.h:1887 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x617/0x690 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810d35c8f8 of 8 bytes by task 496 on cpu 1:
 ext4_writepages+0x134/0x1e10 fs/ext4/inode.c:2658
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x560 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6a0/0x1020 fs/fs-writeback.c:1721
 wb_writeback+0x27d/0x660 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2039
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 496 Comm: kworker/u4:3 Not tainted 5.10.0-rc1-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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