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

[David Hildenbrand (Arm)]

On 3/27/26 13:57, Muhammad Usama Anjum wrote:
> From: Ryan Roberts <ryan.roberts@xxxxxxx>
> 
> 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@xxxxxxx/
> Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Co-developed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> ---
> 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?

-- 
Cheers,

David