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Mon, 16 Mar 2026 15:21:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id p1vYBBAguGkWVQAAD6G6ig (envelope-from ); Mon, 16 Mar 2026 15:21:52 +0000 Message-ID: <220e97f0-dc82-4f37-b833-7160aee46cea@suse.cz> Date: Mon, 16 Mar 2026 16:21:51 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Vlastimil Babka Subject: Re: [PATCH v2 1/3] mm/page_alloc: Optimize free_contig_range() Content-Language: en-US To: Muhammad Usama Anjum , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Uladzislau Rezki , Nick Terrell , David Sterba , "Vishal Moola (Oracle)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Ryan.Roberts@arm.com, david.hildenbrand@arm.com References: <20260316113209.945853-1-usama.anjum@arm.com> <20260316113209.945853-2-usama.anjum@arm.com> Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20260316113209.945853-2-usama.anjum@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2D0C910000E X-Stat-Signature: qxwj91cfhsnds9eoxromaxyji8ha83qi X-Rspam-User: X-HE-Tag: 1773674513-634802 X-HE-Meta: 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 OHw03kcQ hxj26X1cloohpgpnSN8o1etoE/i8YkHG9UyJ2oc1F3ThJW1JFp6ZoiECdDdxpLAtURMqdHB36vPUxlIal76qd9QRTBOFUfyIkocuqXAmiKioo9MhgujgB+wHrJzgesQcebf+w8osEcK6cUq2+7rkMKyG7YiDYLi+xedSFxsC7TNrZ7QH/A/hmLPyCIzLTxEVh5G8IkcNiqgkeZPcUHkbmXM36120crTGnf46eAgEW+JVnEXoDYfRAfjx/28Rg7eGSexF3c01Q3bBPcutJHWE5AxiIVGOTR+tP+iDLjXXJLEuYEy4mSfKfopelWnKsfer+31mXaPy77uUYAKkERoA+vlqwYic6wn9f5r7B4ahYJHRt2l9pErCX46JZMj7fRdWgfkNgtqHj0V6pJW84LOm+yFuM8Nhv2MkCtjAIylhhfgT274LjqP62y0i5O2h2MowtFHqUN486nq4FdM/SI67rUe1fdg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/16/26 12:31, Muhammad Usama Anjum wrote: > From: Ryan Roberts > > 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 > Co-developed-by: Muhammad Usama Anjum > Signed-off-by: Muhammad Usama Anjum > --- > 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..96ac7aae370c4 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 > > +unsigned long __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..6a9430f720579 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -91,6 +91,13 @@ 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. > + * TODO: Perform per-subpage free_pages_prepare() checks for order > 0 pages > + * (HWPoison, PageNetpp, bad free page). > + */ I'm confused, and reading the v1 thread didn't help either. Where would the subpages to check come from? AFAICS we start from order-0 pages always. __free_contig_range calls free_pages_prepare on every page with order 0 unconditionally, so we check every page as an order-0 page. If we then free the bunch of individually checked pages as a high-order page, there's no reason to check those subpages again, no? Am I missing something? > +#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 +1317,9 @@ __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); > @@ -1579,8 +1589,10 @@ static void __free_pages_ok(struct page *page, unsigned int order, > unsigned long pfn = page_to_pfn(page); > struct zone *zone = page_zone(page); > > - if (__free_pages_prepare(page, order, fpi_flags)) > - free_one_page(zone, page, pfn, order, fpi_flags); > + if (!__free_pages_prepare(page, order, fpi_flags)) > + return; > + > + free_one_page(zone, page, pfn, order, fpi_flags); This is not a functional change, can we drop it? > } > > void __meminit __free_pages_core(struct page *page, unsigned int order, > @@ -6784,6 +6796,93 @@ 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 int order; > + unsigned long pfn; > + > + pfn = page_to_pfn(page); > + /* 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(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; > + } > +} > + > +/** > + * __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. > + * > + * Returns the number of pages which were not freed, because their reference > + * count did not fall to zero. We probably don't need this part. > + * > + * Context: May be called in interrupt context or while holding a normal > + * spinlock, but not in NMI context or while holding a raw spinlock. > + */ > +unsigned long __free_contig_range(unsigned long pfn, unsigned long nr_pages) > +{ > + struct page *page = pfn_to_page(pfn); > + unsigned long not_freed = 0; > + struct page *start = NULL; > + 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. > + */ > + 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) > + not_freed++; > + else if (!free_pages_prepare(page, 0)) > + can_free = false; > + > + if (!can_free && start) { > + free_prepared_contig_range(start, page - start); > + start = NULL; > + } else if (can_free && !start) { > + start = page; > + } > + } > + > + if (start) > + free_prepared_contig_range(start, page - start); > + > + return not_freed; > +} > +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) > @@ -7327,11 +7426,14 @@ EXPORT_SYMBOL(free_contig_frozen_range); > */ > void free_contig_range(unsigned long pfn, unsigned long nr_pages) > { > + unsigned long count; > + > if (WARN_ON_ONCE(PageHead(pfn_to_page(pfn)))) > return; > > - for (; nr_pages--; pfn++) > - __free_page(pfn_to_page(pfn)); > + count = __free_contig_range(pfn, nr_pages); > + WARN(count != 0, "%lu pages are still in use!\n", count); And we almost certainly don't want this warning. Spurious temporary page refcount increases (get_page_unless_zero()) can happen e.g. due to memory compaction pfn scanners. It just might mean that side will be then the last one to drop the refcount and freeing the order-0 page. For us it means only that we abort and restart the batching, so we get worse performance, but functionally it's ok, and should be very rare anyway. > + > } > EXPORT_SYMBOL(free_contig_range); > #endif /* CONFIG_CONTIG_ALLOC */