Re: [PATCH v6 2/3] vmalloc: Optimize vfree with free_pages_bulk()

Uladzislau Rezki
2026-04-01T15:13:19Z

On Wed, Apr 01, 2026 at 11:16:20AM +0100, 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: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> 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 v5:
> - Change subject
>
> Changes since v4:
> - Use num_pages_contiguous() instead of raw loop
>
> 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 | 28 ++++++++++++++++++++++++++++
> mm/vmalloc.c | 16 +++++-----------
> 3 files changed, 35 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 fb4522ba51faf..6568af69b5cdb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5175,6 +5175,34 @@ 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)
> +{
> + while (nr_pages) {
> + unsigned long nr_contig = num_pages_contiguous(page_array, nr_pages);
> +
> + __free_contig_range(page_to_pfn(*page_array), nr_contig);
> +
> + nr_pages -= nr_contig;
> + page_array += nr_contig;
> + cond_resched();
> + }
> +}
> +
> /*
> * This is the 'heart' of the zoned buddy allocator.
> */
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c607307c657a6..e9b3d6451e48b 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3459,19 +3459,13 @@ void vfree(const void *addr)
>
> if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
> vm_reset_perms(vm);
> - for (i = 0; i < vm->nr_pages; i++) {
> - struct page *page = vm->pages[i];
>
> - BUG_ON(!page);
> - /*
> - * High-order allocs for huge vmallocs are split, so
> - * can be freed as an array of order-0 allocations
> - */
> - if (!(vm->flags & VM_MAP_PUT_PAGES))
> - mod_lruvec_page_state(page, NR_VMALLOC, -1);
> - __free_page(page);
> - cond_resched();
> + if (!(vm->flags & VM_MAP_PUT_PAGES)) {
> + for (i = 0; i < vm->nr_pages; i++)
> + mod_lruvec_page_state(vm->pages[i], NR_VMALLOC, -1);
> }
> + free_pages_bulk(vm->pages, vm->nr_pages);
> +
> kvfree(vm->pages);
> kfree(vm);
> }
> --
> 2.47.3
>

LGTM: At least vmalloc change

I do not see any concern(mentioned by AI) if mod_lruvec_page_state();
is invoked in loop without cond_resched(). The operation is light and
nr_pages can not be millions of pages.

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

--
Uladzislau Rezki