Re: [PATCH v4 2/3] vmalloc: Optimize vfree

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

On 30/03/2026 3:38 pm, David Hildenbrand (Arm) wrote:
> On 3/27/26 13:57, Muhammad Usama Anjum wrote:
>> From: Ryan Roberts <ryan.roberts@arm.com>
>>
>> Whenever vmalloc allocates high order pages (e.g. for a huge mapping) it
>> must immediately split_page() to order-0 so that it remains compatible
>> with users that want to access the underlying struct page.
>> Commit a06157804399 ("mm/vmalloc: request large order pages from buddy
>> allocator") recently made it much more likely for vmalloc to allocate
>> high order pages which are subsequently split to order-0.
>>
>> Unfortunately this had the side effect of causing performance
>> regressions for tight vmalloc/vfree loops (e.g. test_vmalloc.ko
>> benchmarks). See Closes: tag. This happens because the high order pages
>> must be gotten from the buddy but then because they are split to
>> order-0, when they are freed they are freed to the order-0 pcp.
>> Previously allocation was for order-0 pages so they were recycled from
>> the pcp.
>>
>> It would be preferable if when vmalloc allocates an (e.g.) order-3 page
>> that it also frees that order-3 page to the order-3 pcp, then the
>> regression could be removed.
>>
>> So let's do exactly that; update stats separately first as coalescing is
>> hard to do correctly without complexity. Use free_pages_bulk() which uses
>> the new __free_contig_range() API to batch-free contiguous ranges of pfns.
>> This not only removes the regression, but significantly improves
>> performance of vfree beyond the baseline.
>>
>> A selection of test_vmalloc benchmarks running on arm64 server class
>> system. mm-new is the baseline. Commit a06157804399 ("mm/vmalloc: request
>> large order pages from buddy allocator") was added in v6.19-rc1 where we
>> see regressions. Then with this change performance is much better. (>0
>> is faster, <0 is slower, (R)/(I) = statistically significant
>> Regression/Improvement):
>>
>> +-----------------+----------------------------------------------------------+-------------------+--------------------+
>> | Benchmark       | Result Class                                             |   mm-new          |  this series       |
>> +=================+==========================================================+===================+====================+
>> | micromm/vmalloc | fix_align_alloc_test: p:1, h:0, l:500000 (usec)          |        1331843.33 |         (I) 67.17% |
>> |                 | fix_size_alloc_test: p:1, h:0, l:500000 (usec)           |         415907.33 |             -5.14% |
>> |                 | fix_size_alloc_test: p:4, h:0, l:500000 (usec)           |         755448.00 |         (I) 53.55% |
>> |                 | fix_size_alloc_test: p:16, h:0, l:500000 (usec)          |        1591331.33 |         (I) 57.26% |
>> |                 | fix_size_alloc_test: p:16, h:1, l:500000 (usec)          |        1594345.67 |         (I) 68.46% |
>> |                 | fix_size_alloc_test: p:64, h:0, l:100000 (usec)          |        1071826.00 |         (I) 79.27% |
>> |                 | fix_size_alloc_test: p:64, h:1, l:100000 (usec)          |        1018385.00 |         (I) 84.17% |
>> |                 | fix_size_alloc_test: p:256, h:0, l:100000 (usec)         |        3970899.67 |         (I) 77.01% |
>> |                 | fix_size_alloc_test: p:256, h:1, l:100000 (usec)         |        3821788.67 |         (I) 89.44% |
>> |                 | fix_size_alloc_test: p:512, h:0, l:100000 (usec)         |        7795968.00 |         (I) 82.67% |
>> |                 | fix_size_alloc_test: p:512, h:1, l:100000 (usec)         |        6530169.67 |        (I) 118.09% |
>> |                 | full_fit_alloc_test: p:1, h:0, l:500000 (usec)           |         626808.33 |             -0.98% |
>> |                 | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         532145.67 |             -1.68% |
>> |                 | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         537032.67 |             -0.96% |
>> |                 | long_busy_list_alloc_test: p:1, h:0, l:500000 (usec)     |        8805069.00 |         (I) 74.58% |
>> |                 | pcpu_alloc_test: p:1, h:0, l:500000 (usec)               |         500824.67 |              4.35% |
>> |                 | random_size_align_alloc_test: p:1, h:0, l:500000 (usec)  |        1637554.67 |         (I) 76.99% |
>> |                 | random_size_alloc_test: p:1, h:0, l:500000 (usec)        |        4556288.67 |         (I) 72.23% |
>> |                 | vm_map_ram_test: p:1, h:0, l:500000 (usec)               |         107371.00 |             -0.70% |
>> +-----------------+----------------------------------------------------------+-------------------+--------------------+
>>
>> Fixes: a06157804399 ("mm/vmalloc: request large order pages from buddy allocator")
>> Closes: https://lore.kernel.org/all/66919a28-bc81-49c9-b68f-dd7c73395a0d@arm.com/
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> 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:
>> - Add kerneldoc comment and update description
>> - Add tag
>>
>> Changes since v2:
>> - Remove BUG_ON in favour of simple implementation as this has never
>>   been seen to output any bug in the past as well
>> - Move the free loop to separate function, free_pages_bulk()
>> - Update stats, lruvec_stat in separate loop
>>
>> Changes since v1:
>> - Rebase on mm-new
>> - Rerun benchmarks
>> ---
>>  include/linux/gfp.h |  2 ++
>>  mm/page_alloc.c     | 38 ++++++++++++++++++++++++++++++++++++++
>>  mm/vmalloc.c        | 16 +++++-----------
>>  3 files changed, 45 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>> index 7c1f9da7c8e56..71f9097ab99a0 100644
>> --- a/include/linux/gfp.h
>> +++ b/include/linux/gfp.h
>> @@ -239,6 +239,8 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
>>  				struct page **page_array);
>>  #define __alloc_pages_bulk(...)			alloc_hooks(alloc_pages_bulk_noprof(__VA_ARGS__))
>>  
>> +void free_pages_bulk(struct page **page_array, unsigned long nr_pages);
>> +
>>  unsigned long alloc_pages_bulk_mempolicy_noprof(gfp_t gfp,
>>  				unsigned long nr_pages,
>>  				struct page **page_array);
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 18a96b51aa0be..64be8a9019dca 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5175,6 +5175,44 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
>>  }
>>  EXPORT_SYMBOL_GPL(alloc_pages_bulk_noprof);
>>  
>> +/*
>> + * free_pages_bulk - Free an array of order-0 pages
>> + * @page_array: Array of pages to free
>> + * @nr_pages: The number of pages in the array
>> + *
>> + * Free the order-0 pages. Adjacent entries whose PFNs form a contiguous
>> + * run are released with a single __free_contig_range() call.
>> + *
>> + * This assumes page_array is sorted in ascending PFN order. Without that,
>> + * the function still frees all pages, but contiguous runs may not be
>> + * detected and the freeing pattern can degrade to freeing one page at a
>> + * time.
>> + *
>> + * Context: Sleepable process context only; calls cond_resched()
>> + */
>> +void free_pages_bulk(struct page **page_array, unsigned long nr_pages)
>> +{
>> +	unsigned long start_pfn = 0, pfn;
>> +	unsigned long i, nr_contig = 0;
>> +
>> +	for (i = 0; i < nr_pages; i++) {
>> +		pfn = page_to_pfn(page_array[i]);
>> +		if (!nr_contig) {
>> +			start_pfn = pfn;
>> +			nr_contig = 1;
>> +		} else if (start_pfn + nr_contig != pfn) {
>> +			__free_contig_range(start_pfn, nr_contig);
>> +			start_pfn = pfn;
>> +			nr_contig = 1;
>> +			cond_resched();
>> +		} else {
>> +			nr_contig++;
>> +		}
>> +	}
> 
> What happened to the idea of using num_pages_contiguous()? I think that
> should generate more efficient code (all we're doing is comparing
> pointers really on SPARSEMEM_VMEMMAP) and the end result looks more
> readable?
Sorry, I misunderstood you in the light of duplicate usage of
num_pages_contiguous(). I'll update the implementation.

Copying from previous thread.

>>> Could we use num_pages_contiguous() here?
>>>
>>> while (nr_pages) {
>>> 	unsigned long nr_contig_pages = num_pages_contiguous(page_array, nr_pages);
>>>
>>> 	__free_contig_range(pfn_to_page(*page_array), nr_contig_pages);
>>> 	
>>> 	nr_pages -= nr_contig;
>>> 	page_array += nr_contig;
>>> 	cond_resched();
>>> }
>>>
>>> Something like that?
>> __free_contig_range() is already checking for the sections. If
>> num_pages_contiguous() is called here, it'll cause the duplication
>> of the section check.
> No problem. For configs we care about it's optimized out entirely either
> way.

Thanks,
Usama