[PATCH 6.1 059/280] mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free

Greg Kroah-Hartman
2026-02-04T15:06:22Z

From: Aboorva Devarajan <aboorvad@linux.ibm.com>

commit b9efe36b5e3eb2e91aa3d706066428648af034fc upstream.

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: Aboorva Devarajan <aboorvad@linux.ibm.com>
Acked-by: Michal 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/page_alloc.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -9067,11 +9067,19 @@ int percpu_pagelist_high_fraction_sysctl
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 */