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

[Muhammad Usama Anjum]

On 24/03/2026 5:14 pm, Zi Yan wrote:
> On 24 Mar 2026, at 11:22, David Hildenbrand wrote:
> 
>> 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@xxxxxxx>
>>>>
>>>> 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@xxxxxxx>
>>>> Co-developed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>>>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>>>> ---
>>>> 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);
>>>                 }
>>>         }
I'll simplify in the next version. Thanks.

>>>
>>> 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() :)
I'll refer to num_pages_contiguous() as well.

> 
> Oh, I needed to refresh my memory on SPARSEMEM to remember
> !SECTION_IN_PAGE_FLAGS is for SPARSE_VMEMMAP and the contiguous PFNs vs
> contiguous struct page thing.
> 
> Now memdesc_section() makes sense to me. Thanks.
> 
> 
> Best Regards,
> Yan, Zi