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

[Michal Hocko <mhocko@suse.com>]

On Mon 01-12-25 11:30:09, Aboorva Devarajan 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.
> 
> 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>

Looks OK. I would just add a short comment explaining that in the code.
See below.

Acked-by: Michal Hocko <mhocko@suse.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;
>  
	/*
	 * Avoid using pcp_batch_high_lock for reads as the value is
	 * read atomicaly and 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 */
> -- 
> 2.50.1

-- 
Michal Hocko
SUSE Labs