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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Uladzislau Rezki , Nick Terrell , David Sterba , Vishal Moola , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Ryan.Roberts@arm.com, david.hildenbrand@arm.com Subject: Re: [PATCH v3 1/3] mm/page_alloc: Optimize free_contig_range() Date: Tue, 24 Mar 2026 10:46:20 -0400 X-Mailer: MailMate (2.0r6290) Message-ID: In-Reply-To: <20260324133538.497616-2-usama.anjum@arm.com> References: <20260324133538.497616-1-usama.anjum@arm.com> <20260324133538.497616-2-usama.anjum@arm.com> Content-Type: text/plain X-ClientProxiedBy: BYAPR02CA0056.namprd02.prod.outlook.com (2603:10b6:a03:54::33) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|SA5PPFAB8DFE4E8:EE_ X-MS-Office365-Filtering-Correlation-Id: cd0a2efe-0ad6-4749-cb47-08de89b42052 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014|22082099003|56012099003|18002099003; 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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 > Co-developed-by: Muhammad Usama Anjum > Signed-off-by: Muhammad Usama Anjum > --- > 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(-) > > + > +/** > + * __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? > + > + if (start) > + free_prepared_contig_range(start, page - start); > +} > +EXPORT_SYMBOL(__free_contig_range); > + > #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 +7424,7 @@ 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 */ > -- > 2.47.3 Best Regards, Yan, Zi