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@linux.ibm.com>
---
 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

On Mon,  1 Dec 2025 11:30:09 +0530 Aboorva Devarajan <aboorvad@linux.ibm.com> wrote:

> 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.

That's pretty bad behavior.

I wonder if there are other problems which can be caused by this
lengthy hold time.

It would be better to address the lengthy hold time rather that having
to work around it in one impacted site.

> 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.
> 
> ...
>
> --- 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 */

That being said, I'll grab the patch and shall put a cc:stable on it,
see what people think about this hold-time issue.