Re: [PATCH v3 1/3] mm/page_alloc: Optimize free_contig_range()

[Muhammad Usama Anjum]

On 24/03/2026 8:56 pm, David Hildenbrand (Arm) wrote:
> 
>> +void __free_contig_range(unsigned long pfn, unsigned long nr_pages)
>> +{
>> +	struct page *page = pfn_to_page(pfn);
>> +	struct page *start = NULL;
>> +	unsigned long start_sec;
>> +	unsigned long i;
>> +	bool can_free;
>> +
>> +	/*
>> +	 * Chunk the range into contiguous runs of pages for which the refcount
>> +	 * went to zero and for which free_pages_prepare() succeeded. If
>> +	 * free_pages_prepare() fails we consider the page to have been freed;
>> +	 * deliberately leak it.
>> +	 *
>> +	 * Code assumes contiguous PFNs have contiguous struct pages, but not
>> +	 * vice versa. Break batches at section boundaries since pages from
>> +	 * different sections must not be coalesced into a single high-order
>> +	 * block.
> 
> The comment is not completely accurate: section boundary only applies to
> some kernel configs.
> 
> Maybe rewrite the whole paragraph into
> 
> "Contiguous PFNs might not have a contiguous "struct pages" in some
> kernel config. Therefore, check memdesc_section(), and stop batching
> once it changes, see num_pages_contiguous()."
Agreed, I'll update.

> 
>> +	 */
>> +	for (i = 0; i < nr_pages; i++, page++) {
>> +		VM_WARN_ON_ONCE(PageHead(page));
>> +		VM_WARN_ON_ONCE(PageTail(page));
>> +
>> +		can_free = put_page_testzero(page);
>> +		if (can_free && !free_pages_prepare(page, 0))
>> +			can_free = false;
>> +
>> +		if (can_free && start &&
>> +		    memdesc_section(page->flags) != start_sec) {
>> +			free_prepared_contig_range(start, page - start);
>> +			start = page;
>> +			start_sec = memdesc_section(page->flags);
>> +		} else if (!can_free && start) {
>> +			free_prepared_contig_range(start, page - start);
>> +			start = NULL;
>> +		} else if (can_free && !start) {
>> +			start = page;
>> +			start_sec = memdesc_section(page->flags);
>> +		}
>> +	}
> 
> Simplification a proposed by Zi make sense to me!
I've added it.

Thanks,
Usama