Re: [PATCH v4 1/3] mm/page_alloc: Optimize free_contig_range()

[Muhammad Usama Anjum <usama.anjum@arm.com>]

On 30/03/2026 3:30 pm, Vlastimil Babka (SUSE) wrote:
> On 3/27/26 13:57, 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 v3:
>> - Move __free_contig_range() to more generic __free_contig_range_common()
>>   which will used to free frozen pages as well
>> - Simplify the loop in __free_contig_range_common()
>> - Rewrite the comment
>>
>> 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
>> ---
>>  include/linux/gfp.h |   2 +
>>  mm/page_alloc.c     | 103 +++++++++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 103 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>> index f82d74a77cad8..7c1f9da7c8e56 100644
>> --- a/include/linux/gfp.h
>> +++ b/include/linux/gfp.h
>> @@ -467,6 +467,8 @@ void free_contig_frozen_range(unsigned long pfn, unsigned long nr_pages);
>>  void free_contig_range(unsigned long pfn, unsigned long nr_pages);
>>  #endif
>>  
>> +void __free_contig_range(unsigned long pfn, unsigned long nr_pages);
>> +
>>  DEFINE_FREE(free_page, void *, free_page((unsigned long)_T))
>>  
>>  #endif /* __LINUX_GFP_H */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 75ee81445640b..18a96b51aa0be 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -91,6 +91,9 @@ typedef int __bitwise fpi_t;
>>  /* Free the page without taking locks. Rely on trylock only. */
>>  #define FPI_TRYLOCK		((__force fpi_t)BIT(2))
>>  
>> +/* free_pages_prepare() has already been called for page(s) being freed. */
>> +#define FPI_PREPARED		((__force fpi_t)BIT(3))
>> +
>>  /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
>>  static DEFINE_MUTEX(pcp_batch_high_lock);
>>  #define MIN_PERCPU_PAGELIST_HIGH_FRACTION (8)
>> @@ -1310,6 +1313,9 @@ __always_inline bool __free_pages_prepare(struct page *page,
> 
> Hm I noticed the function isn't static, but it should be, and this is a good
> oportunity to make it so.
I'll make it static in the v5.

> 
>>  	bool compound = PageCompound(page);
>>  	struct folio *folio = page_folio(page);
>>  
>> +	if (fpi_flags & FPI_PREPARED)
>> +		return true;
>> +
>>  	VM_BUG_ON_PAGE(PageTail(page), page);
>>  
>>  	trace_mm_page_free(page, order);
> 
> ...
> 
>> +/**
>> + * __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)
>> +{
>> +	__free_contig_range_common(pfn, nr_pages, false);
>> +}
>> +EXPORT_SYMBOL(__free_contig_range);
> 
> I don't think the export is necessary for anything? Please drop.
No, I'll drop it.

> 
>> +
>>  #ifdef CONFIG_CONTIG_ALLOC
>>  /* Usage: See admin-guide/dynamic-debug-howto.rst */
>>  static void alloc_contig_dump_pages(struct list_head *page_list)
>> @@ -7330,8 +7430,7 @@ void free_contig_range(unsigned long pfn, unsigned long nr_pages)
>>  	if (WARN_ON_ONCE(PageHead(pfn_to_page(pfn))))
>>  		return;
>>  
>> -	for (; nr_pages--; pfn++)
>> -		__free_page(pfn_to_page(pfn));
>> +	__free_contig_range(pfn, nr_pages);
>>  }
>>  EXPORT_SYMBOL(free_contig_range);
>>  #endif /* CONFIG_CONTIG_ALLOC */
>