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Message-ID: <4a66f13d-318b-4cdb-b168-0c993ff8a309@arm.com>
Date: Wed, 17 Dec 2025 17:01:19 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Vishal Moola <vishal.moola@...il.com>, Dev Jain <dev.jain@....com>,
Baoquan He <bhe@...hat.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter
On 17/12/2025 15:20, Ryan Roberts wrote:
> On 17/12/2025 12:02, Uladzislau Rezki wrote:
>>> On 16/12/2025 21:19, Uladzislau Rezki (Sony) wrote:
>>>> Introduce a module parameter to enable or disable the large-order
>>>> allocation path in vmalloc. High-order allocations are disabled by
>>>> default so far, but users may explicitly enable them at runtime if
>>>> desired.
>>>>
>>>> High-order pages allocated for vmalloc are immediately split into
>>>> order-0 pages and later freed as order-0, which means they do not
>>>> feed the per-CPU page caches. As a result, high-order attempts tend
>>>> to bypass the PCP fastpath and fall back to the buddy allocator that
>>>> can affect performance.
>>>>
>>>> However, when the PCP caches are empty, high-order allocations may
>>>> show better performance characteristics especially for larger
>>>> allocation requests.
>>>
>>> I wonder if a better solution would be "allocate order-0 if available in pcp,
>>> else try large order, else fallback to order-0" Could that provide the best of
>>> all worlds without needing a configuration knob?
>>>
>> I am not sure, to me it looks like a bit odd.
>
> Perhaps it would feel better if it was generalized to "first try allocation from
> PCP list, highest to lowest order, then try allocation from the buddy, highest
> to lowest order"?
>
>> Ideally it would be
>> good just free it as high-order page and not order-0 peaces.
>
> Yeah perhaps that's better. How about something like this (very lightly tested
> and no performance results yet):
>
> (And I should admit I'm not 100% sure it is safe to call free_frozen_pages()
> with a contiguous run of order-0 pages, but I'm not seeing any warnings or
> memory leaks when running mm selftests...)
>
> ---8<---
> commit caa3e5eb5bfade81a32fa62d1a8924df1eb0f619
> Author: Ryan Roberts <ryan.roberts@....com>
> Date: Wed Dec 17 15:11:08 2025 +0000
>
> WIP
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index b155929af5b1..d25f5b867e6b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -383,6 +383,8 @@ extern void __free_pages(struct page *page, unsigned int order);
> extern void free_pages_nolock(struct page *page, unsigned int order);
> extern void free_pages(unsigned long addr, unsigned int order);
>
> +void free_pages_bulk(struct page *page, int nr_pages);
> +
> #define __free_page(page) __free_pages((page), 0)
> #define free_page(addr) free_pages((addr), 0)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 822e05f1a964..5f11224cf353 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5304,6 +5304,48 @@ static void ___free_pages(struct page *page, unsigned int
> order,
> }
> }
>
> +static void free_frozen_pages_bulk(struct page *page, int nr_pages)
> +{
> + while (nr_pages) {
> + unsigned int fit_order, align_order, order;
> + unsigned long pfn;
> +
> + pfn = page_to_pfn(page);
> + fit_order = ilog2(nr_pages);
> + align_order = pfn ? __ffs(pfn) : fit_order;
> + order = min3(fit_order, align_order, MAX_PAGE_ORDER);
> +
> + free_frozen_pages(page, order);
> +
> + page += 1U << order;
> + nr_pages -= 1U << order;
> + }
> +}
> +
> +void free_pages_bulk(struct page *page, int nr_pages)
> +{
> + struct page *start = NULL;
> + bool can_free;
> + int i;
> +
> + for (i = 0; i < nr_pages; i++, page++) {
> + VM_BUG_ON_PAGE(PageHead(page), page);
> + VM_BUG_ON_PAGE(PageTail(page), page);
> +
> + can_free = put_page_testzero(page);
> +
> + if (!can_free && start) {
> + free_frozen_pages_bulk(start, page - start);
> + start = NULL;
> + } else if (can_free && !start) {
> + start = page;
> + }
> + }
> +
> + if (start)
> + free_frozen_pages_bulk(start, page - start);
> +}
> +
> /**
> * __free_pages - Free pages allocated with alloc_pages().
> * @page: The page pointer returned from alloc_pages().
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index ecbac900c35f..8f782bac1ece 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3429,7 +3429,8 @@ void vfree_atomic(const void *addr)
> void vfree(const void *addr)
> {
> struct vm_struct *vm;
> - int i;
> + struct page *start;
> + int i, nr;
>
> if (unlikely(in_interrupt())) {
> vfree_atomic(addr);
> @@ -3455,17 +3456,26 @@ void vfree(const void *addr)
> /* All pages of vm should be charged to same memcg, so use first one. */
> if (vm->nr_pages && !(vm->flags & VM_MAP_PUT_PAGES))
> mod_memcg_page_state(vm->pages[0], MEMCG_VMALLOC, -vm->nr_pages);
> - for (i = 0; i < vm->nr_pages; i++) {
> +
> + start = vm->pages[0];
> + BUG_ON(!start);
> + nr = 1;
> + for (i = 1; 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
> - */
> - __free_page(page);
> - cond_resched();
> +
> + if (start + nr != page) {
> + free_pages_bulk(start, nr);
> + start = page;
> + nr = 1;
> + cond_resched();
> + } else {
> + nr++;
> + }
> }
> + free_pages_bulk(start, nr);
> +
> if (!(vm->flags & VM_MAP_PUT_PAGES))
> atomic_long_sub(vm->nr_pages, &nr_vmalloc_pages);
> kvfree(vm->pages);
> ---8<---
I tested this on a performance monitoring system and see a huge improvement for
the test_vmalloc tests.
Both columns are compared to v6.18. 6-19-0-rc1 has Vishal's change to allocate
large orders, which I previously reported the regressions for. vfree-high-order
adds the above patch to free contiguous order-0 pages in bulk.
(R)/(I) means statistically significant regression/improvement. Results are
normalized so that less than zero is regression and greater than zero is
improvement.
+-----------------+----------------------------------------------------------+--------------+------------------+
| Benchmark | Result Class | 6-19-0-rc1 | vfree-high-order |
+=================+==========================================================+==============+==================+
| micromm/vmalloc | fix_align_alloc_test: p:1, h:0, l:500000 (usec) | (R) -40.69% | (I) 3.98% |
| | fix_size_alloc_test: p:1, h:0, l:500000 (usec) | 0.10% | -1.47% |
| | fix_size_alloc_test: p:4, h:0, l:500000 (usec) | (R) -22.74% | (I) 11.57% |
| | fix_size_alloc_test: p:16, h:0, l:500000 (usec) | (R) -23.63% | (I) 47.42% |
| | fix_size_alloc_test: p:16, h:1, l:500000 (usec) | -1.58% | (I) 106.01% |
| | fix_size_alloc_test: p:64, h:0, l:100000 (usec) | (R) -24.39% | (I) 99.12% |
| | fix_size_alloc_test: p:64, h:1, l:100000 (usec) | (I) 2.34% | (I) 196.87% |
| | fix_size_alloc_test: p:256, h:0, l:100000 (usec) | (R) -23.29% | (I) 125.42% |
| | fix_size_alloc_test: p:256, h:1, l:100000 (usec) | (I) 3.74% | (I) 238.59% |
| | fix_size_alloc_test: p:512, h:0, l:100000 (usec) | (R) -23.80% | (I) 132.38% |
| | fix_size_alloc_test: p:512, h:1, l:100000 (usec) | (R) -2.84% | (I) 514.75% |
| | full_fit_alloc_test: p:1, h:0, l:500000 (usec) | 2.74% | 0.33% |
| | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | 0.58% | 1.36% |
| | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) | -0.66% | 1.48% |
| | long_busy_list_alloc_test: p:1, h:0, l:500000 (usec) | (R) -25.24% | (I) 77.95% |
| | pcpu_alloc_test: p:1, h:0, l:500000 (usec) | -0.58% | 0.60% |
| | random_size_align_alloc_test: p:1, h:0, l:500000 (usec) | (R) -45.75% | (I) 8.51% |
| | random_size_alloc_test: p:1, h:0, l:500000 (usec) | (R) -28.16% | (I) 65.34% |
| | vm_map_ram_test: p:1, h:0, l:500000 (usec) | -0.54% | -0.33% |
+-----------------+----------------------------------------------------------+--------------+------------------+
What do you think?
Thanks,
Ryan
>
>>
>>>>
>>>> Since the best strategy is workload-dependent, this patch adds a
>>>> parameter letting users to choose whether vmalloc should try
>>>> high-order allocations or stay strictly on the order-0 fastpath.
>>>>
>>>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
>>>> ---
>>>> mm/vmalloc.c | 9 +++++++--
>>>> 1 file changed, 7 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>>>> index d3a4725e15ca..f66543896b16 100644
>>>> --- a/mm/vmalloc.c
>>>> +++ b/mm/vmalloc.c
>>>> @@ -43,6 +43,7 @@
>>>> #include <asm/tlbflush.h>
>>>> #include <asm/shmparam.h>
>>>> #include <linux/page_owner.h>
>>>> +#include <linux/moduleparam.h>
>>>>
>>>> #define CREATE_TRACE_POINTS
>>>> #include <trace/events/vmalloc.h>
>>>> @@ -3671,6 +3672,9 @@ vm_area_alloc_pages_large_order(gfp_t gfp, int nid, unsigned int order,
>>>> return nr_allocated;
>>>> }
>>>>
>>>> +static int attempt_larger_order_alloc;
>>>> +module_param(attempt_larger_order_alloc, int, 0644);
>>>
>>> Would this be better as a bool? Docs say that you can then specify 0/1, y/n or
>>> Y/N as the value; that's probably more intuitive?
>>>
>>> nit: I'd favour a shorter name. Perhaps large_order_alloc?
>>>
>> Thanks! We can switch to bool and use shorter name for sure.
>>
>> --
>> Uladzislau Rezki
>
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