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BUG: KCSAN: data-race in __xa_set_mark / find_get_pages_range_tag

write to 0xffff88811f3e6228 of 8 bytes by task 18365 on cpu 0:
 instrument_write include/linux/instrumented.h:42 [inline]
 __test_and_set_bit include/asm-generic/bitops/instrumented-non-atomic.h:71 [inline]
 node_set_mark lib/xarray.c:93 [inline]
 xas_set_mark lib/xarray.c:879 [inline]
 __xa_set_mark+0x233/0x330 lib/xarray.c:1708
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_write_end+0x13d/0x220 fs/buffer.c:2162
 generic_write_end+0x5c/0x250 fs/buffer.c:2176
 ext4_da_write_end+0x57e/0x760 fs/ext4/inode.c:3091
 generic_perform_write+0x23b/0x390 mm/filemap.c:3512
 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 0xffff88811f3e6228 of 8 bytes by task 21 on cpu 1:
 xas_find_chunk include/linux/xarray.h:1625 [inline]
 xas_next_marked include/linux/xarray.h:1656 [inline]
 find_get_pages_range_tag+0x30c/0x5b0 mm/filemap.c:2074
 pagevec_lookup_range_tag+0x37/0x50 mm/swap.c:1122
 mpage_prepare_extent_to_map+0x19c/0x650 fs/ext4/inode.c:2558
 ext4_writepages+0x97d/0x1ed0 fs/ext4/inode.c:2779
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x600 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6c0/0xff0 fs/fs-writeback.c:1721
 __writeback_inodes_wb+0xb0/0x2b0 fs/fs-writeback.c:1790
 wb_writeback+0x2b0/0x6f0 fs/fs-writeback.c:1896
 wb_check_start_all fs/fs-writeback.c:2020 [inline]
 wb_do_writeback+0x423/0x5f0 fs/fs-writeback.c:2046
 wb_workfn+0xc8/0x470 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x9a0 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:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 21 Comm: kworker/u4:1 Not tainted 5.9.0-rc6-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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