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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff88811e9c3cd0 of 8 bytes by task 14358 on cpu 1:
 __add_to_page_cache_locked+0x2cc/0x710 mm/filemap.c:868
 add_to_page_cache_lru+0xa0/0x1b0 mm/filemap.c:919
 pagecache_get_page+0x570/0x970 mm/filemap.c:1721
 grab_cache_page_write_begin+0x3f/0x70 mm/filemap.c:3353
 ext4_da_write_begin+0x358/0xad0 fs/ext4/inode.c:2965
 generic_perform_write+0x196/0x390 mm/filemap.c:3403
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:269
 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

read to 0xffff88811e9c3cd0 of 8 bytes by task 14362 on cpu 0:
 ext4_writepages+0x145/0x1ef0 fs/ext4/inode.c:2649
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:422
 filemap_write_and_wait_range+0x8b/0x2a0 mm/filemap.c:655
 filemap_write_and_wait include/linux/fs.h:2630 [inline]
 iomap_bmap+0x68/0x110 fs/iomap/fiemap.c:130
 ext4_bmap+0x20b/0x220 fs/ext4/inode.c:3214
 bmap+0x56/0x80 fs/inode.c:1703
 ioctl_fibmap fs/ioctl.c:74 [inline]
 file_ioctl fs/ioctl.c:531 [inline]
 do_vfs_ioctl+0xd44/0xec0 fs/ioctl.c:732
 __do_sys_ioctl fs/ioctl.c:751 [inline]
 __se_sys_ioctl+0x83/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 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: 14362 Comm: syz-executor.5 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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