mm: vmscan: remove deadlock due to throttling failing to make progress

In the Linux kernel, the following vulnerability has been resolved:

mm: vmscan: remove deadlock due to throttling failing to make progress

A soft lockup bug in kcompactd was reported in a private bugzilla with
the following visible in dmesg;

watchdog: BUG: soft lockup - CPU#33 stuck for 26s! [kcompactd0:479]
watchdog: BUG: soft lockup - CPU#33 stuck for 52s! [kcompactd0:479]
watchdog: BUG: soft lockup - CPU#33 stuck for 78s! [kcompactd0:479]
watchdog: BUG: soft lockup - CPU#33 stuck for 104s! [kcompactd0:479]

The machine had 256G of RAM with no swap and an earlier failed
allocation indicated that node 0 where kcompactd was run was potentially
unreclaimable;

Node 0 active_anon:29355112kB inactive_anon:2913528kB active_file:0kB
inactive_file:0kB unevictable:64kB isolated(anon):0kB isolated(file):0kB
mapped:8kB dirty:0kB writeback:0kB shmem:26780kB shmem_thp:
0kB shmem_pmdmapped: 0kB anon_thp: 23480320kB writeback_tmp:0kB
kernel_stack:2272kB pagetables:24500kB all_unreclaimable? yes

Vlastimil Babka investigated a crash dump and found that a task
migrating pages was trying to drain PCP lists;

PID: 52922 TASK: ffff969f820e5000 CPU: 19 COMMAND: "kworker/u128:3"
Call Trace:
__schedule
schedule
schedule_timeout
wait_for_completion
__flush_work
__drain_all_pages
__alloc_pages_slowpath.constprop.114
__alloc_pages
alloc_migration_target
migrate_pages
migrate_to_node
do_migrate_pages
cpuset_migrate_mm_workfn
process_one_work
worker_thread
kthread
ret_from_fork

This failure is specific to CONFIG_PREEMPT=n builds. The root of the
problem is that kcompact0 is not rescheduling on a CPU while a task that
has isolated a large number of the pages from the LRU is waiting on
kcompact0 to reschedule so the pages can be released. While
shrink_inactive_list() only loops once around too_many_isolated, reclaim
can continue without rescheduling if sc->skipped_deactivate == 1 which
could happen if there was no file LRU and the inactive anon list was not
low.