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 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 6e8c79ea62f1c..9218fda8842a6 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
Nit: the subject could be more specific, e.g. like this?
vmalloc: Optimize vfree with free_pages_bulk()
On 3/31/26 17:22, 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>
> +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);
I'll note that num_pages_contiguous() already handled crossing the section
boundaries and __free_contig_range() checks them again. But that's fine I
think, we don't have to optimize for !SPARSEMEM_VMEMMAP architectures and on
SPARSEMEM_VMEMMAP the checks should be compiled out, right.
It would complicate the API otherwise.
> +
> + nr_pages -= nr_contig;
> + page_array += nr_contig;
> + cond_resched();
> + }
> +}
> +
> /*
> * This is the 'heart' of the zoned buddy allocator.
> */
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