Re: [PATCH v3 2/2] memblock: use binary search to locate candidate regions

[Mike Rapoport <rppt@kernel.org>]

On Sat, Sep 26, 2026 at 09:24:48AM +0000, Tarun Sahu wrote:
> Use binary search (memblock_bsearch_start) in memblock_add_range() and
> memblock_isolate_range() to locate candidate regions instead of linearly
> scanning from index 0.
> 
> Under heavy memory fragmentation (such as KHO page preservation registering
> 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 worst-case complexity to O(N log N)
> (and O(N) for sequential appends), 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@google.com>
> 
>  mm/memblock.c | 53 +++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 37 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 59dda7d085f3..87c71435c80c 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -586,6 +586,33 @@ static void __init_memblock memblock_insert_region(struct memblock_type *type,
>  	type->total_size += size;
>  }
>  
> +/**
> + * memblock_bsearch_start - 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.
> + */
> +static int __init_memblock memblock_bsearch_start(struct memblock_type *type,
> +						  phys_addr_t base)

I'd call it __memblock_search()

> +{
> +	int mid, low = 0;
> +	int high = type->cnt;
> +
> +	if (type->cnt && base >= type->regions[type->cnt - 1].base +
> +				 type->regions[type->cnt - 1].size)
> +		return type->cnt;

Using local variables would make it more readable IMHO.

> +
> +	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
> @@ -609,7 +636,7 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>  	bool insert = false;
>  	phys_addr_t obase = base;
>  	phys_addr_t end = base + memblock_cap_size(base, &size);
> -	int idx, nr_new, start_rgn = -1, end_rgn;
> +	int idx, start_idx, nr_new, start_rgn = -1, end_rgn;
>  
>  	if (!size)
>  		return 0;
> @@ -644,8 +671,9 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
>  	 */
>  	base = obase;
>  	nr_new = 0;
> +	start_idx = memblock_bsearch_start(type, base);
>  
> -	for (idx = 0; idx < type->cnt; idx++) {
> +	for (idx = start_idx; idx < type->cnt; idx++) {
>  		struct memblock_region *rgn = &type->regions[idx];
>  		phys_addr_t rbase = rgn->base;
>  		phys_addr_t rend = rbase + rgn->size;
> @@ -809,7 +837,7 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
>  					int *start_rgn, int *end_rgn)
>  {
>  	phys_addr_t end = base + memblock_cap_size(base, &size);
> -	int idx;
> +	int idx, start_idx;
>  
>  	*start_rgn = *end_rgn = 0;
>  
> @@ -821,7 +849,9 @@ 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++) {
> +	start_idx = memblock_bsearch_start(type, base);
> +
> +	for (idx = start_idx; idx < type->cnt; idx++) {
>  		struct memblock_region *rgn = &type->regions[idx];
>  		phys_addr_t rbase = rgn->base;
>  		phys_addr_t rend = rbase + rgn->size;
> @@ -2062,19 +2092,10 @@ 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;
> +	int idx = memblock_bsearch_start(type, addr);
>  
> -	do {
> -		unsigned int mid = (right + left) / 2;
> -
> -		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;
>  }
>  
> base-commit: 1f18d740165163910df64d3063e1ad31648bc5e0
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
> 

-- 
Sincerely yours,
Mike.