mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free

When page isolation loops indefinitely during memory offline, reading
/proc/sys/vm/percpu_pagelist_high_fraction blocks on pcp_batch_high_lock,
causing hung task warnings.

Make procfs reads lock-free since percpu_pagelist_high_fraction is a
simple integer with naturally atomic reads, writers still serialize via
the mutex.

This prevents hung task warnings when reading the procfs file during
long-running memory offline operations.

[[e-mail]: add comment, per Michal]
Link: https://lkml.kernel.org/r/aS_y9AuJQFydLEXo@tiehlicka
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Aboorva Devarajan <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Brendan Jackman <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Zi Yan <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/page_alloc.c

···
 	int old_percpu_pagelist_high_fraction;
 	int ret;
 
+	/*
+	 * Avoid using pcp_batch_high_lock for reads as the value is read
+	 * atomically and a race with offlining is harmless.
+	 */
+
+	if (!write)
+		return proc_dointvec_minmax(table, write, buffer, length, ppos);
+
 	mutex_lock(&pcp_batch_high_lock);
 	old_percpu_pagelist_high_fraction = percpu_pagelist_high_fraction;
 
 	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
-	if (!write || ret < 0)
+	if (ret < 0)
 		goto out;
 
 	/* Sanity checking to avoid pcp imbalance */