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BUG: KCSAN: data-race in compact_zone / isolate_migratepages_block

write to 0xffff88813fffbc10 of 8 bytes by task 3389 on cpu 1:
 update_cached_migrate mm/compaction.c:434 [inline]
 isolate_migratepages_block+0x1964/0x1c90 mm/compaction.c:1122
 isolate_migratepages mm/compaction.c:1892 [inline]
 compact_zone+0x1597/0x1e60 mm/compaction.c:2342
 compact_node+0xe4/0x1e0 mm/compaction.c:2641
 compact_nodes mm/compaction.c:2657 [inline]
 sysctl_compaction_handler+0x47/0x90 mm/compaction.c:2678
 proc_sys_call_handler+0x3dd/0x530 fs/proc/proc_sysctl.c:591
 proc_sys_write+0x1e/0x30 fs/proc/proc_sysctl.c:617
 call_write_iter include/linux/fs.h:1977 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f9/0x7e0 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+0x44/0xae

read to 0xffff88813fffbc10 of 8 bytes by task 3395 on cpu 0:
 update_cached_migrate mm/compaction.c:433 [inline]
 isolate_migratepages mm/compaction.c:1887 [inline]
 compact_zone+0x1491/0x1e60 mm/compaction.c:2342
 compact_node+0xe4/0x1e0 mm/compaction.c:2641
 compact_nodes mm/compaction.c:2657 [inline]
 sysctl_compaction_handler+0x47/0x90 mm/compaction.c:2678
 proc_sys_call_handler+0x3dd/0x530 fs/proc/proc_sysctl.c:591
 proc_sys_write+0x1e/0x30 fs/proc/proc_sysctl.c:617
 call_write_iter include/linux/fs.h:1977 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f9/0x7e0 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+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 3395 Comm: syz-executor.2 Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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