Re: [PATCH 1/3] mm/page_alloc: Do not calculate node's total pages and memmap pages when empty

[David Hildenbrand <david@redhat.com>]

On 07.03.22 16:07, Oscar Salvador wrote:
> free_area_init_node() calls calculate_node_totalpages() and
> free_area_init_core(). The former to get node's {spanned,present}_pages,
> and the latter to calculate, among other things, how many pages per zone
> we spent on memmap_pages, which is used to substract zone's free pages.
> 
> On memoryless-nodes, it is pointless to perform such a bunch of work, so
> make sure we skip the calculations when having a node or empty zone.
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>

Sorry, I'm late with review. My mailbox got flooded.

> ---
>  mm/page_alloc.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 967085c1c78a..0b7d176a8990 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7312,6 +7312,10 @@ static void __init calculate_node_totalpages(struct pglist_data *pgdat,
>  	unsigned long realtotalpages = 0, totalpages = 0;
>  	enum zone_type i;
>  
> +	/* Skip calculation for memoryless nodes */
> +	if (node_start_pfn == node_end_pfn)
> +		goto no_pages;
> +

Just a NIT:

E.g., in zone_spanned_pages_in_node() we test for
	!node_start_pfn && !node_end_pfn

In update_pgdat_span(), we set
	node_start_pfn = node_end_pfn = 0;
when we find an empty node during memory unplug.

Therefore, I wonder if a helper "is_memoryless_node()" or "node_empty()"
might be reasonable, that just checks for either
	!node_start_pfn && !node_end_pfn
or
	node_start_pfn == node_end_pfn



>  	for (i = 0; i < MAX_NR_ZONES; i++) {
>  		struct zone *zone = pgdat->node_zones + i;
>  		unsigned long zone_start_pfn, zone_end_pfn;
> @@ -7344,6 +7348,7 @@ static void __init calculate_node_totalpages(struct pglist_data *pgdat,
>  		realtotalpages += real_size;
>  	}
>  
> +no_pages:
>  	pgdat->node_spanned_pages = totalpages;
>  	pgdat->node_present_pages = realtotalpages;
>  	pr_debug("On node %d totalpages: %lu\n", pgdat->node_id, realtotalpages);
> @@ -7562,6 +7567,10 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
>  		size = zone->spanned_pages;
>  		freesize = zone->present_pages;
>  
> +		/* No pages? Nothing to calculate then. */
> +		if (!size)
> +			goto no_pages;
> +
>  		/*
>  		 * Adjust freesize so that it accounts for how much memory
>  		 * is used by this zone for memmap. This affects the watermark
> @@ -7597,6 +7606,7 @@ static void __init free_area_init_core(struct pglist_data *pgdat)
>  		 * when the bootmem allocator frees pages into the buddy system.
>  		 * And all highmem pages will be managed by the buddy system.
>  		 */
> +no_pages:
>  		zone_init_internals(zone, j, nid, freesize);
>  
>  		if (!size)

We have another size check below. We essentially have right now:

"
	if (!size)
		goto no_pages;

	[code]
no_pages:
	zone_init_internals(zone, j, nid, freesize);

	if (!size)
		continue
	[more code]
"

IMHO, it would be nicer to avoid the label/goto by just doing a:

"
	if (!size) {
		zone_init_internals(zone, j, nid, 0);
		continue;
	}

	[code]
	zone_init_internals(zone, j, nid, freesize);
	[more code]
"

Or factoring out [code] into a separate function.


Anyhow, the change itself looks sane.

-- 
Thanks,

David / dhildenb