Re: [PATCH v5 1/3] mm/page_alloc: Optimize free_contig_range()

[Zi Yan <ziy@nvidia.com>]

On 31 Mar 2026, at 11:21, 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 v4:
> - Move can_free initialization inside the loop
> - Make __free_pages_prepare() static on reviewer's request
> - Remove export of __free_contig_range
> - Use pfn_to_page() for each pfn instead of page++
>
> 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     | 110 ++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 108 insertions(+), 4 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..6e8c79ea62f1c 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)
> @@ -1301,8 +1304,8 @@ static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
>
>  #endif /* CONFIG_MEM_ALLOC_PROFILING */
>
> -__always_inline bool __free_pages_prepare(struct page *page,
> -					  unsigned int order, fpi_t fpi_flags)
> +static __always_inline bool __free_pages_prepare(struct page *page,
> +		unsigned int order, fpi_t fpi_flags)
>  {
>  	int bad = 0;
>  	bool skip_kasan_poison = should_skip_kasan_poison(page);
> @@ -1310,6 +1313,9 @@ __always_inline bool __free_pages_prepare(struct page *page,
>  	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);
> @@ -6784,6 +6790,103 @@ void __init page_alloc_sysctl_init(void)
>  	register_sysctl_init("vm", page_alloc_sysctl_table);
>  }
>
> +static void free_prepared_contig_range(struct page *page,
> +		unsigned long nr_pages)
> +{
> +	while (nr_pages) {
> +		unsigned long pfn = page_to_pfn(page);

pfn does not change after this assignment. That is why David suggested
prefixing a const. You can send a fixup to this patch to change this
if there is no substantial change needed for this series.

> +		unsigned int order;
> +
> +		/* We are limited by the largest buddy order. */
> +		order = pfn ? __ffs(pfn) : MAX_PAGE_ORDER;
> +		/* Don't exceed the number of pages to free. */
> +		order = min_t(unsigned int, order, ilog2(nr_pages));
> +		order = min_t(unsigned int, order, MAX_PAGE_ORDER);
> +
> +		/*
> +		 * Free the chunk as a single block. Our caller has already
> +		 * called free_pages_prepare() for each order-0 page.
> +		 */
> +		__free_frozen_pages(page, order, FPI_PREPARED);
> +
> +		page += 1UL << order;
> +		nr_pages -= 1UL << order;
> +	}
> +}
> +
> +static void __free_contig_range_common(unsigned long pfn, unsigned long nr_pages,
> +		bool is_frozen)
> +{
> +	struct page *page, *start = NULL;
> +	unsigned long nr_start = 0;
> +	unsigned long start_sec;
> +	unsigned long i;
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		bool can_free = true;
> +
> +		/*
> +		 * Contiguous PFNs might not have contiguous "struct pages"
> +		 * in some kernel configs: page++ across a section boundary
> +		 * is undefined. Use pfn_to_page() for each PFN.
> +		 */
> +		page = pfn_to_page(pfn + i);

page is local to this loop. You probably can move its declaration here.
But feel free to ignore this suggestion.

I was about to suggest make it const, but put_page_test_zero()
and free_pages_prepare() do not accept const struct page yet.

> +
> +		VM_WARN_ON_ONCE(PageHead(page));
> +		VM_WARN_ON_ONCE(PageTail(page));
> +
> +		if (!is_frozen)
> +			can_free = put_page_testzero(page);
> +
> +		if (can_free)
> +			can_free = free_pages_prepare(page, 0);
> +
> +		if (!can_free) {
> +			if (start) {
> +				free_prepared_contig_range(start, i - nr_start);
> +				start = NULL;
> +			}
> +			continue;
> +		}
> +
> +		if (start && memdesc_section(page->flags) != start_sec) {
> +			free_prepared_contig_range(start, i - nr_start);
> +			start = page;
> +			nr_start = i;
> +			start_sec = memdesc_section(page->flags);
> +		} else if (!start) {
> +			start = page;
> +			nr_start = i;
> +			start_sec = memdesc_section(page->flags);
> +		}
> +	}
> +
> +	if (start)
> +		free_prepared_contig_range(start, nr_pages - nr_start);
> +}
> +

Otherwise, LGTM. Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi