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

Muhammad Usama Anjum
2026-03-31T15:10:35Z

On 30/03/2026 3:35 pm, Vlastimil Babka (SUSE) 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>
>
> Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Thank you. But as the implementation of free_pages_bulk() has
changed to using num_pages_contiguous(), I cannot take this tag.
Please review the next version again.

>
>

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Thanks,
Usama