Re: [PATCH v7 2/2] mm/memory hotplug/unplug: Optimize zone->contiguous update when changes pfn range

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/22/25 15:58, Tianyou Li wrote:
> When invoke move_pfn_range_to_zone or remove_pfn_range_from_zone, it will
> update the zone->contiguous by checking the new zone's pfn range from the
> beginning to the end, regardless the previous state of the old zone. When
> the zone's pfn range is large, the cost of traversing the pfn range to
> update the zone->contiguous could be significant.
> 
> Add fast paths to quickly detect cases where zone is definitely not
> contiguous without scanning the new zone. The cases are: when the new range
> did not overlap with previous range, the contiguous should be false; if the
> new range adjacent with the previous range, just need to check the new
> range; if the new added pages could not fill the hole of previous zone, the
> contiguous should be false.
> 
> The following test cases of memory hotplug for a VM [1], tested in the
> environment [2], show that this optimization can significantly reduce the
> memory hotplug time [3].
> 
> +----------------+------+---------------+--------------+----------------+
> |                | Size | Time (before) | Time (after) | Time Reduction |
> |                +------+---------------+--------------+----------------+
> | Plug Memory    | 256G |      10s      |      2s      |       80%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      33s      |      6s      |       81%      |
> +----------------+------+---------------+--------------+----------------+
> 
> +----------------+------+---------------+--------------+----------------+
> |                | Size | Time (before) | Time (after) | Time Reduction |
> |                +------+---------------+--------------+----------------+
> | Unplug Memory  | 256G |      10s      |      2s      |       80%      |
> |                +------+---------------+--------------+----------------+
> |                | 512G |      34s      |      6s      |       82%      |
> +----------------+------+---------------+--------------+----------------+
> 

Again, very nice results.

[...]

>   
> +static enum zone_contig_state zone_contig_state_after_shrinking(struct zone *zone,
> +				unsigned long start_pfn, unsigned long nr_pages)
> +{
> +	const unsigned long end_pfn = start_pfn + nr_pages;
> +
> +	/*
> +	 * If the removed pfn range inside the original zone span, the contiguous
> +	 * property is surely false.

/*
  * If we cut a hole into the zone span, then the zone is
  * certainly not contiguous.
  */

> +	 */
> +	if (start_pfn > zone->zone_start_pfn && end_pfn < zone_end_pfn(zone))
> +		return ZONE_CONTIG_NO;
> +
> +	/* If the removed pfn range is at the beginning or end of the
> +	 * original zone span, the contiguous property is preserved when
> +	 * the original zone is contiguous.

/* Removing from the start/end of the zone will not change anything. */

> +	 */
> +	if (start_pfn == zone->zone_start_pfn || end_pfn == zone_end_pfn(zone))
> +		return zone->contiguous ? ZONE_CONTIG_YES : ZONE_CONTIG_MAYBE;
> +
> +	return ZONE_CONTIG_MAYBE;
> +}
> +
>   void remove_pfn_range_from_zone(struct zone *zone,
>   				      unsigned long start_pfn,
>   				      unsigned long nr_pages)
> @@ -551,6 +573,7 @@ void remove_pfn_range_from_zone(struct zone *zone,
>   	const unsigned long end_pfn = start_pfn + nr_pages;
>   	struct pglist_data *pgdat = zone->zone_pgdat;
>   	unsigned long pfn, cur_nr_pages;
> +	enum zone_contig_state new_contiguous_state = ZONE_CONTIG_MAYBE;

No need to initialize, given that you overwrite the value below.

>   
>   	/* Poison struct pages because they are now uninitialized again. */
>   	for (pfn = start_pfn; pfn < end_pfn; pfn += cur_nr_pages) {
> @@ -571,12 +594,14 @@ void remove_pfn_range_from_zone(struct zone *zone,
>   	if (zone_is_zone_device(zone))
>   		return;
>   
> +	new_contiguous_state = zone_contig_state_after_shrinking(zone, start_pfn,
> +								 nr_pages);
>   	clear_zone_contiguous(zone);
>   
>   	shrink_zone_span(zone, start_pfn, start_pfn + nr_pages);
>   	update_pgdat_span(pgdat);
>   
> -	set_zone_contiguous(zone);
> +	set_zone_contiguous(zone, new_contiguous_state);
>   }
>   
>   /**
> @@ -736,6 +761,39 @@ static inline void section_taint_zone_device(unsigned long pfn)
>   }
>   #endif
>   
> +static enum zone_contig_state zone_contig_state_after_growing(struct zone *zone,
> +				unsigned long start_pfn, unsigned long nr_pages)
> +{
> +	const unsigned long end_pfn = start_pfn + nr_pages;
> +
> +	if (zone_is_empty(zone))
> +		return ZONE_CONTIG_YES;
> +
> +	/*
> +	 * If the moved pfn range does not intersect with the original zone spa

s/spa/span/

> +	 * the contiguous property is surely false.

"the zone is surely not contiguous."

> +	 */
> +	if (end_pfn < zone->zone_start_pfn || start_pfn > zone_end_pfn(zone))
> +		return ZONE_CONTIG_NO;
> +
> +	/*
> +	 * If the moved pfn range is adjacent to the original zone span, given
> +	 * the moved pfn range's contiguous property is always true, the zone's
> +	 * contiguous property inherited from the original value.
> +	 */

/* Adding to the start/end of the zone will not change anything. */

> +	if (end_pfn == zone->zone_start_pfn || start_pfn == zone_end_pfn(zone))
> +		return zone->contiguous ? ZONE_CONTIG_YES : ZONE_CONTIG_NO;
> +
> +	/*
> +	 * If the original zone's hole larger than the moved pages in the range
> +	 * the contiguous property is surely false.
> +	 */

/* If we cannot fill the hole, the zone stays not contiguous. */

> +	if (nr_pages < (zone->spanned_pages - zone->present_pages))
> +		return ZONE_CONTIG_NO;
> +
> +	return ZONE_CONTIG_MAYBE;
> +}
> +
>   /*
>    * Associate the pfn range with the given zone, initializing the memmaps
>    * and resizing the pgdat/zone data to span the added pages. After this
> @@ -1090,11 +1148,20 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
>   {
>   	unsigned long end_pfn = pfn + nr_pages;
>   	int ret, i;
> +	enum zone_contig_state new_contiguous_state = ZONE_CONTIG_NO;
>   
>   	ret = kasan_add_zero_shadow(__va(PFN_PHYS(pfn)), PFN_PHYS(nr_pages));
>   	if (ret)
>   		return ret;
>   
> +	/*
> +	 * If the allocated memmap pages are not in a full section, keep the
> +	 * contiguous state as ZONE_CONTIG_NO.
> +	 */
> +	if (IS_ALIGNED(end_pfn, PAGES_PER_SECTION))
> +		new_contiguous_state = zone_contig_state_after_growing(zone,
> +								pfn, nr_pages);
> +

This is nasty. I would wish we could just leave that code path alone.

In particular: I am 99% sure that we never ever run into this case in 
practice.

E.g., on x86, we can have up to 2 GiB memory blocks. But the memmap of 
that is 64/4096*2GiB == 32 MB ... and a memory section is 128 MiB.


As commented on patch #1, we should drop the set_zone_contiguous() in 
this function either way and let online_pages() deal with it.

We just have to make sure that we don't create some inconsistencies by 
doing that.

Can you double-check?

>   	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_UNMOVABLE,
>   			       false);
>   
> @@ -1113,7 +1180,7 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
>   	if (nr_pages >= PAGES_PER_SECTION)
>   	        online_mem_sections(pfn, ALIGN_DOWN(end_pfn, PAGES_PER_SECTION));
>   
> -	set_zone_contiguous(zone);
> +	set_zone_contiguous(zone, new_contiguous_state);
>   	return ret;
>   }
>   
> @@ -1153,6 +1220,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>   	const int nid = zone_to_nid(zone);
>   	int need_zonelists_rebuild = 0;
>   	unsigned long flags;
> +	enum zone_contig_state new_contiguous_state = ZONE_CONTIG_NO;
>   	int ret;
>   
>   	/*
> @@ -1166,6 +1234,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>   			 !IS_ALIGNED(pfn + nr_pages, PAGES_PER_SECTION)))
>   		return -EINVAL;
>   
> +	new_contiguous_state = zone_contig_state_after_growing(zone, pfn, nr_pages);
>   
>   	/* associate pfn range with the zone */
>   	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_MOVABLE,
> @@ -1204,7 +1273,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>   	}
>   
>   	online_pages_range(pfn, nr_pages);
> -	set_zone_contiguous(zone);
> +	set_zone_contiguous(zone, new_contiguous_state);
>   	adjust_present_page_count(pfn_to_page(pfn), group, nr_pages);
>   
>   	if (node_arg.nid >= 0)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index fc2a6f1e518f..0c41f1004847 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2263,11 +2263,19 @@ void __init init_cma_pageblock(struct page *page)
>   }
>   #endif
>   
> -void set_zone_contiguous(struct zone *zone)
> +void set_zone_contiguous(struct zone *zone, enum zone_contig_state state)
>   {
>   	unsigned long block_start_pfn = zone->zone_start_pfn;
>   	unsigned long block_end_pfn;
>   
> +	if (state == ZONE_CONTIG_YES) {
> +		zone->contiguous = true;
> +		return;
> +	}
> +

Maybe add a comment like

/* We expect an earlier call to clear_zone_contig(). */


And maybe move that comment all the way up in the function and add

VM_WARN_ON_ONCE(zone->contiguous);

> +	if (state == ZONE_CONTIG_NO)
> +		return;
> +
>   	block_end_pfn = pageblock_end_pfn(block_start_pfn);
>   	for (; block_start_pfn < zone_end_pfn(zone);
>   			block_start_pfn = block_end_pfn,
> @@ -2348,7 +2356,7 @@ void __init page_alloc_init_late(void)
>   		shuffle_free_memory(NODE_DATA(nid));
>   
>   	for_each_populated_zone(zone)
> -		set_zone_contiguous(zone);
> +		set_zone_contiguous(zone, ZONE_CONTIG_MAYBE);
>   
>   	/* Initialize page ext after all struct pages are initialized. */
>   	if (deferred_struct_pages)


-- 
Cheers

David