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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff8880bb784800 of 8 bytes by task 17896 on cpu 0:
 __add_to_page_cache_locked+0x2cc/0x6e0 mm/filemap.c:867
 add_to_page_cache_lru+0xa0/0x1a0 mm/filemap.c:918
 page_cache_readahead_unbounded+0x234/0x440 mm/readahead.c:228
 ondemand_readahead+0x4e4/0x6e0 mm/readahead.c:273
 page_cache_async_readahead+0x1a6/0x1c0 mm/readahead.c:631
 generic_file_buffered_read+0x303/0x1750 mm/filemap.c:2041
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2307
 ext4_file_read_iter+0x2dd/0x410 fs/ext4/file.c:73
 call_read_iter include/linux/fs.h:1901 [inline]
 generic_file_splice_read+0x22a/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:870 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x562/0xb10 fs/read_write.c:1521
 __do_sys_sendfile64 fs/read_write.c:1582 [inline]
 __se_sys_sendfile64 fs/read_write.c:1568 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1568
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bb784800 of 8 bytes by task 17893 on cpu 1:
 ext4_writepages+0x132/0x1cb0 fs/ext4/inode.c:2650
 do_writepages+0x7b/0x150 mm/page-writeback.c:2354
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:421
 file_write_and_wait_range+0x9f/0x120 mm/filemap.c:760
 ext4_sync_file+0xe2/0x680 fs/ext4/fsync.c:151
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2882 [inline]
 ext4_buffered_write_iter+0x34e/0x390 fs/ext4/file.c:277
 ext4_file_write_iter+0x4a3/0xfd0 fs/ext4/file.c:498
 call_write_iter include/linux/fs.h:1907 [inline]
 do_iter_readv_writev+0x321/0x3c0 fs/read_write.c:694
 do_iter_write+0x10e/0x470 fs/read_write.c:999
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1040
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x562/0xb10 fs/read_write.c:1521
 __do_sys_sendfile64 fs/read_write.c:1576 [inline]
 __se_sys_sendfile64 fs/read_write.c:1568 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1568
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 17893 Comm: syz-executor.2 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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