Re: [PATCH v5 02/12] mm/page_alloc: keep the pagesets of an empty zone

[Zi Yan <ziy@nvidia.com>]

On 5 Nov 2026, at 4:58, Vlastimil Babka (SUSE) wrote:

> On 11/3/26 17:04, Zi Yan wrote:
>> On Tue Nov 3, 2026 at 9:57 AM EST, Vlastimil Babka (SUSE) wrote:
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index f1048168c90c..1850a41f24f7 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -6308,6 +6308,10 @@ void __meminit setup_zone_pageset(struct zone *zone)
>>>>>  {
>>>>>  	int cpu;
>>>>>
>>>>> +	/* The zone has been populated before, see zone_pcp_reset(). */
>>>>> +	if (zone->per_cpu_pageset != &boot_pageset)
>>>>> +		return;
>>>>> +
>>>>>  	/* Size may be 0 on !SMP && !NUMA */
>>>>>  	if (sizeof(struct per_cpu_zonestat) > 0)
>>>>>  		zone->per_cpu_zonestats = alloc_percpu(struct per_cpu_zonestat);
>>>>> @@ -6400,6 +6404,10 @@ void __init setup_per_cpu_pageset(void)
>>>>>
>>>>>  __meminit void zone_pcp_init(struct zone *zone)
>>>>>  {
>>>>> +	/* A node which is added again, see zone_pcp_reset(). */
>>>>
>>>> it might be better to explain zone->per_cpu_pageset can only be NULL at
>>>> boot time and it needs an initialization.
>>>
>>> Hmm.
>>>
>>>>> +	if (zone->per_cpu_pageset && zone->per_cpu_pageset != &boot_pageset)
>>>>> +		return;
>>>
>>> Shouldn't then the check be just:
>>>
>>> if (zone->per_cpu_pageset)
>>> 	return;
>>>
>>> Because if it's == &boot_pageset, we are just setting the same values that
>>> area already there?
>>>
>>
>> Right. And zone_pcp_init(), called by zone_init_internals(), becomes a nop for
>> free_area_init_core_hotplug(). Then I wonder if zone_init_internals() is
>> needed for hotplug, but that belongs to a later cleanup.
>
> Yep, that's for later cleanup. Right now the minimum that's correct and not
> unnecessarily misleading is enough. So I've done this:
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1850a41f24f7..20bfd11c5851 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6404,8 +6404,8 @@ void __init setup_per_cpu_pageset(void)
>
>  __meminit void zone_pcp_init(struct zone *zone)
>  {
> -       /* A node which is added again, see zone_pcp_reset(). */
> -       if (zone->per_cpu_pageset && zone->per_cpu_pageset != &boot_pageset)
> +       /* This initialization only needs to be done once */
> +       if (zone->per_cpu_pageset)
>                 return;
>
>         /*
LGTM. Thanks.

Best Regards,
Yan, Zi