Commit b9efe36b authored by Aboorva Devarajan's avatar Aboorva Devarajan Committed by Andrew Morton
Browse files

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.

[akpm@linux-foundation.org: add comment, per Michal]
  Link: https://lkml.kernel.org/r/aS_y9AuJQFydLEXo@tiehlicka
Link: https://lkml.kernel.org/r/20251201060009.1420792-1-aboorvad@linux.ibm.com


Signed-off-by: default avatarAboorva Devarajan <aboorvad@linux.ibm.com>
Acked-by: default avatarMichal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent a38be546

mm/page_alloc.c

+9 −1
Original line number Diff line number Diff line
@@ -6667,11 +6667,19 @@ static int percpu_pagelist_high_fraction_sysctl_handler(const struct ctl_table * 
	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 */