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

[Aboorva Devarajan]

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.

Signed-off-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
---
 mm/page_alloc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ed82ee55e66a..7c8d773ed4af 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6611,11 +6611,14 @@ static int percpu_pagelist_high_fraction_sysctl_handler(const struct ctl_table *
 	int old_percpu_pagelist_high_fraction;
 	int ret;
 
+	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 */
-- 
2.50.1