[PATCH v4 2/3] vmalloc: Optimize vfree

Muhammad Usama Anjum
2026-03-27T12:57:45Z

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++;
+ }
+ }
+ if (nr_contig)
+ __free_contig_range(start_pfn, nr_contig);
+}
+
/*
* 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