Re: [PATCH -next v2] mm, proc: collect percpu free pages into the free pages

[Andrew Morton]

On Mon, 22 Aug 2022 11:33:54 +0800 Liu Shixin <liushixin2@xxxxxxxxxx> wrote:

> The page on pcplist could be used, but not counted into memory free or
> avaliable, and pcp_free is only showed by show_mem() for now. Since commit
> d8a759b57035 ("mm, page_alloc: double zone's batchsize"), there is a
> significant decrease in the display of free memory, with a large number
> of cpus and zones, the number of pages in the percpu list can be very
> large, so it is better to let user to know the pcp count.
> 
> On a machine with 3 zones and 72 CPUs. Before commit d8a759b57035, the
> maximum amount of pages in the pcp lists was theoretically 162MB(3*72*768KB).
> After the patch, the lists can hold 324MB. It has been observed to be 114MB
> in the idle state after system startup in practice(increased 80 MB).
> 

Seems reasonable.

> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 033f1e26d15b..f89928d3ad4e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5853,6 +5853,26 @@ static unsigned long nr_free_zone_pages(int offset)
>  	return sum;
>  }
>  
> +static unsigned long nr_free_zone_pcplist_pages(struct zone *zone)
> +{
> +	unsigned long sum = 0;
> +	int cpu;
> +
> +	for_each_online_cpu(cpu)
> +		sum += per_cpu_ptr(zone->per_cpu_pageset, cpu)->count;
> +	return sum;
> +}
> +
> +static unsigned long nr_free_pcplist_pages(void)
> +{
> +	unsigned long sum = 0;
> +	struct zone *zone;
> +
> +	for_each_zone(zone)
> +		sum += nr_free_zone_pcplist_pages(zone);
> +	return sum;
> +}

Prevention of races against zone/node hotplug?