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

[David Hildenbrand <david.hildenbrand@arm.com>]

On 3/24/26 15:46, Zi Yan wrote:
> On 24 Mar 2026, at 9:35, Muhammad Usama Anjum wrote:
> 
>> From: Ryan Roberts <ryan.roberts@arm.com>
>>
>> Decompose the range of order-0 pages to be freed into the set of largest
>> possible power-of-2 size and aligned chunks and free them to the pcp or
>> buddy. This improves on the previous approach which freed each order-0
>> page individually in a loop. Testing shows performance to be improved by
>> more than 10x in some cases.
>>
>> Since each page is order-0, we must decrement each page's reference
>> count individually and only consider the page for freeing as part of a
>> high order chunk if the reference count goes to zero. Additionally
>> free_pages_prepare() must be called for each individual order-0 page
>> too, so that the struct page state and global accounting state can be
>> appropriately managed. But once this is done, the resulting high order
>> chunks can be freed as a unit to the pcp or buddy.
>>
>> This significantly speeds up the free operation but also has the side
>> benefit that high order blocks are added to the pcp instead of each page
>> ending up on the pcp order-0 list; memory remains more readily available
>> in high orders.
>>
>> vmalloc will shortly become a user of this new optimized
>> free_contig_range() since it aggressively allocates high order
>> non-compound pages, but then calls split_page() to end up with
>> contiguous order-0 pages. These can now be freed much more efficiently.
>>
>> The execution time of the following function was measured in a server
>> class arm64 machine:
>>
>> static int page_alloc_high_order_test(void)
>> {
>> 	unsigned int order = HPAGE_PMD_ORDER;
>> 	struct page *page;
>> 	int i;
>>
>> 	for (i = 0; i < 100000; i++) {
>> 		page = alloc_pages(GFP_KERNEL, order);
>> 		if (!page)
>> 			return -1;
>> 		split_page(page, order);
>> 		free_contig_range(page_to_pfn(page), 1UL << order);
>> 	}
>>
>> 	return 0;
>> }
>>
>> Execution time before: 4097358 usec
>> Execution time after:   729831 usec
>>
>> Perf trace before:
>>
>>     99.63%     0.00%  kthreadd         [kernel.kallsyms]      [.] kthread
>>             |
>>             ---kthread
>>                0xffffb33c12a26af8
>>                |
>>                |--98.13%--0xffffb33c12a26060
>>                |          |
>>                |          |--97.37%--free_contig_range
>>                |          |          |
>>                |          |          |--94.93%--___free_pages
>>                |          |          |          |
>>                |          |          |          |--55.42%--__free_frozen_pages
>>                |          |          |          |          |
>>                |          |          |          |           --43.20%--free_frozen_page_commit
>>                |          |          |          |                     |
>>                |          |          |          |                      --35.37%--_raw_spin_unlock_irqrestore
>>                |          |          |          |
>>                |          |          |          |--11.53%--_raw_spin_trylock
>>                |          |          |          |
>>                |          |          |          |--8.19%--__preempt_count_dec_and_test
>>                |          |          |          |
>>                |          |          |          |--5.64%--_raw_spin_unlock
>>                |          |          |          |
>>                |          |          |          |--2.37%--__get_pfnblock_flags_mask.isra.0
>>                |          |          |          |
>>                |          |          |           --1.07%--free_frozen_page_commit
>>                |          |          |
>>                |          |           --1.54%--__free_frozen_pages
>>                |          |
>>                |           --0.77%--___free_pages
>>                |
>>                 --0.98%--0xffffb33c12a26078
>>                           alloc_pages_noprof
>>
>> Perf trace after:
>>
>>      8.42%     2.90%  kthreadd         [kernel.kallsyms]         [k] __free_contig_range
>>             |
>>             |--5.52%--__free_contig_range
>>             |          |
>>             |          |--5.00%--free_prepared_contig_range
>>             |          |          |
>>             |          |          |--1.43%--__free_frozen_pages
>>             |          |          |          |
>>             |          |          |           --0.51%--free_frozen_page_commit
>>             |          |          |
>>             |          |          |--1.08%--_raw_spin_trylock
>>             |          |          |
>>             |          |           --0.89%--_raw_spin_unlock
>>             |          |
>>             |           --0.52%--free_pages_prepare
>>             |
>>              --2.90%--ret_from_fork
>>                        kthread
>>                        0xffffae1c12abeaf8
>>                        0xffffae1c12abe7a0
>>                        |
>>                         --2.69%--vfree
>>                                   __free_contig_range
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> Co-developed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
>> ---
>> Changes since v2:
>> - Handle different possible section boundries in __free_contig_range()
>> - Drop the TODO
>> - Remove return value from __free_contig_range()
>> - Remove non-functional change from __free_pages_ok()
>>
>> Changes since v1:
>> - Rebase on mm-new
>> - Move FPI_PREPARED check inside __free_pages_prepare() now that
>>   fpi_flags are already being passed.
>> - Add todo (Zi Yan)
>> - Rerun benchmarks
>> - Convert VM_BUG_ON_PAGE() to VM_WARN_ON_ONCE()
>> - Rework order calculation in free_prepared_contig_range() and use
>>   MAX_PAGE_ORDER as high limit instead of pageblock_order as it must
>>   be up to internal __free_frozen_pages() how it frees them
>>
>> Made-with: Cursor
>> ---
>>  include/linux/gfp.h |  2 +
>>  mm/page_alloc.c     | 97 ++++++++++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 97 insertions(+), 2 deletions(-)
>>
> 
> <snip>
> 
>> +
>> +/**
>> + * __free_contig_range - Free contiguous range of order-0 pages.
>> + * @pfn: Page frame number of the first page in the range.
>> + * @nr_pages: Number of pages to free.
>> + *
>> + * For each order-0 struct page in the physically contiguous range, put a
>> + * reference. Free any page who's reference count falls to zero. The
>> + * implementation is functionally equivalent to, but significantly faster than
>> + * calling __free_page() for each struct page in a loop.
>> + *
>> + * Memory allocated with alloc_pages(order>=1) then subsequently split to
>> + * order-0 with split_page() is an example of appropriate contiguous pages that
>> + * can be freed with this API.
>> + *
>> + * Context: May be called in interrupt context or while holding a normal
>> + * spinlock, but not in NMI context or while holding a raw spinlock.
>> + */
>> +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.
>> +	 */
>> +	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);
>> +		}
>> +	}
> 
> It can be simplified to:
> 
>         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) && free_pages_prepare(page, 0);
> 
>                 if (!can_free) {
>                         if (start) {
>                                 free_prepared_contig_range(start, page - start);
>                                 start = NULL;
>                         }
>                         continue;
>                 }
> 
>                 if (start && memdesc_section(page->flags) != start_sec) {
>                         free_prepared_contig_range(start, page - start);
>                         start = page;
>                         start_sec = memdesc_section(page->flags);
>                 } else if (!start) {
>                         start = page;
>                         start_sec = memdesc_section(page->flags);
>                 }
>         }
> 
> BTW, memdesc_section() returns 0 for !SECTION_IN_PAGE_FLAGS.
> Is pfn_to_section_nr() more robust?

That's the whole trick: it's optimized out in that case. Linus proposed
that for num_pages_contiguous().

The cover letter should likely refer to num_pages_contiguous() :)

-- 
Cheers,

David