Re: [PATCH v4 2/2] memblock: use binary search to locate candidate regions

[Mike Rapoport]

On Thu, Oct 08, 2026 at 07:08:28PM +0000, Tarun Sahu wrote:
> Use binary search (memblock_bsearch_start) in memblock_add_range() and

The function name is __memblock_search now and I think you can drop it
altogether :)

> memblock_isolate_range() to locate candidate regions instead of linearly
> scanning from index 0.
> 
> Under heavy memory fragmentation (such as KHO page preservation registering

Comma instead of open parenthesis feels better to me.

> hundreds of thousands of disjoint folios), scanning from index 0 on every
> insertion and isolation results in O(N^2) complexity, causing boot-time
> memory retrieval to take several minutes (~268s for 393k pages).
> 
> Using binary search reduces the candidate search complexity to O(log N)
> (from O(N)), cutting KHO memory retrieval time from ~268s to ~50ms.
> 
> memblock_search() open codes the same binary search, so reimplement it on
> top of the new helper.
> 
> Signed-off-by: Tarun Sahu <tarunsahu@xxxxxxxxxx>
> Reviewed-by: Pratyush Yadav <pratyush@xxxxxxxxxx>
> ---
>  mm/memblock.c | 45 +++++++++++++++++++++++++++++++--------------
>  1 file changed, 31 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 59dda7d085f3..614d0a55dd70 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -586,6 +586,29 @@ static void __init_memblock memblock_insert_region(struct memblock_type *type,
>  	type->total_size += size;
>  }
>  
> +/**
> + * __memblock_search - Find the first region index where rend > base
> + * @type: memblock type to search
> + * @base: base physical address of the candidate range
> + *
> + * Returns the first region index that could potentially overlap @base.

Return: for proper kernel-doc format AFAIR

> + */
> +static int __init_memblock __memblock_search(struct memblock_type *type,
> +						  phys_addr_t base)
> +{
> +	int mid, low = 0;
> +	int high = type->cnt;
> +
> +	while (low < high) {
> +		mid = (low + high) / 2;
> +		if (type->regions[mid].base + type->regions[mid].size <= base)
> +			low = mid + 1;
> +		else
> +			high = mid;
> +	}
> +	return low;
> +}
> +
>  /**
>   * memblock_add_range - add new memblock region
>   * @type: memblock type to add new region into
> @@ -644,8 +667,9 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>  	 */
>  	base = obase;
>  	nr_new = 0;
> +	idx = __memblock_search(type, base);
>  
> -	for (idx = 0; idx < type->cnt; idx++) {
> +	for (; idx < type->cnt; idx++) {


	for (idx = __memblock_search(type, base); ...

>  		struct memblock_region *rgn = &type->regions[idx];
>  		phys_addr_t rbase = rgn->base;
>  		phys_addr_t rend = rbase + rgn->size;
> @@ -821,7 +845,9 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
>  		if (memblock_double_array(type, base, size) < 0)
>  			return -ENOMEM;
>  
> -	for (idx = 0; idx < type->cnt; idx++) {
> +	idx = __memblock_search(type, base);
> +
> +	for (; idx < type->cnt; idx++) {

Same here or even

	for (int idx = __memblock_search(type, base); ...

>  		struct memblock_region *rgn = &type->regions[idx];
>  		phys_addr_t rbase = rgn->base;
>  		phys_addr_t rend = rbase + rgn->size;
> @@ -2062,19 +2088,10 @@ void __init memblock_mem_limit_remove_map(phys_addr_t limit)
>  
>  static int __init_memblock memblock_search(struct memblock_type *type, phys_addr_t addr)
>  {
> -	unsigned int left = 0, right = type->cnt;
> -
> -	do {
> -		unsigned int mid = (right + left) / 2;
> +	int idx = __memblock_search(type, addr);
>  
> -		if (addr < type->regions[mid].base)
> -			right = mid;
> -		else if (addr >= (type->regions[mid].base +
> -				  type->regions[mid].size))
> -			left = mid + 1;
> -		else
> -			return mid;
> -	} while (left < right);
> +	if (idx < type->cnt && addr >= type->regions[idx].base)
> +		return idx;
>  	return -1;
>  }
>  
> -- 
> 2.56.0.385.gd3acb90ef8-goog
> 

-- 
Sincerely yours,
Mike.