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Message-ID: <aUMDCSTewPSLCbYM@milan>
Date: Wed, 17 Dec 2025 20:22:49 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Uladzislau Rezki <urezki@...il.com>, 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 Wed, Dec 17, 2025 at 05:01:19PM +0000, Ryan Roberts wrote:
> 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?
>
You were first :)
Some figures from me:
# Default(3 pages)
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 541868 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 542515 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 541561 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 542951 usec
# Patch(3 pages)
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 585266 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 594301 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 598912 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 589345 usec
Now the perf figures are almost settled and aligned with default!
We do use per-cpu-cache for 3 pages allocations.
# Default(100 pages)
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 5724919 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 5721430 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 5717224 usec
# Patch(100 pages)
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2629600 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2622811 usec
fix_size_alloc_test passed: 1 failed: 0 xfailed: 0 repeat: 1 loops: 1000000 avg: 2629324 usec
~2x faster! It is because of freeing now occurs much more efficient
so we spent less cycles on free path comparing with default case.
See below, perf also confirms that vfree() ~2x consumes less cycles:
# Default
+ 96.99% 0.49% [test_vmalloc] [k] fix_size_alloc_test
+ 59.64% 2.38% [kernel] [k] vfree.part.0
+ 45.69% 15.80% [kernel] [k] __free_frozen_pages
+ 39.83% 0.00% [kernel] [k] ret_from_fork_asm
+ 39.83% 0.00% [kernel] [k] ret_from_fork
+ 39.83% 0.00% [kernel] [k] kthread
+ 38.67% 0.00% [test_vmalloc] [k] test_func
+ 36.64% 0.01% [kernel] [k] __vmalloc_node_noprof
+ 36.63% 0.20% [kernel] [k] __vmalloc_node_range_noprof
+ 17.55% 4.94% [kernel] [k] alloc_pages_bulk_noprof
+ 16.46% 12.21% [kernel] [k] free_frozen_page_commit.isra.0
+ 16.06% 8.09% [kernel] [k] vmap_small_pages_range_noflush
+ 12.56% 10.82% [kernel] [k] __rmqueue_pcplist
+ 9.45% 9.43% [kernel] [k] __get_pfnblock_flags_mask.isra.0
+ 7.95% 7.95% [kernel] [k] pfn_valid
+ 5.77% 0.03% [kernel] [k] remove_vm_area
+ 5.44% 5.44% [kernel] [k] ___free_pages
+ 4.67% 4.59% [kernel] [k] __vunmap_range_noflush
+ 4.30% 4.30% [kernel] [k] __list_add_valid_or_report
# Patch
+ 94.28% 1.00% [test_vmalloc] [k] fix_size_alloc_test
+ 55.63% 0.03% [kernel] [k] __vmalloc_node_noprof
+ 55.60% 3.78% [kernel] [k] __vmalloc_node_range_noprof
+ 37.26% 19.29% [kernel] [k] vmap_small_pages_range_noflush
+ 37.12% 5.63% [kernel] [k] vfree.part.0
+ 30.59% 0.00% [kernel] [k] ret_from_fork_asm
+ 30.59% 0.00% [kernel] [k] ret_from_fork
+ 30.59% 0.00% [kernel] [k] kthread
+ 28.79% 0.00% [test_vmalloc] [k] test_func
+ 17.90% 17.88% [kernel] [k] pfn_valid
+ 13.24% 0.02% [kernel] [k] remove_vm_area
+ 10.90% 10.68% [kernel] [k] __vunmap_range_noflush
+ 10.81% 10.80% [kernel] [k] free_pages_bulk
+ 7.09% 0.51% [kernel] [k] alloc_pages_noprof
+ 6.58% 0.41% [kernel] [k] alloc_pages_mpol
+ 6.50% 0.30% [kernel] [k] free_frozen_pages_bulk
+ 5.74% 0.97% [kernel] [k] __alloc_frozen_pages_noprof
+ 5.70% 0.00% [kernel] [k] worker_thread
+ 5.62% 0.02% [kernel] [k] process_one_work
+ 5.57% 0.01% [kernel] [k] __purge_vmap_area_lazy
+ 4.76% 2.55% [kernel] [k] get_page_from_freelist
So it is nice :)
--
Uladzislau Rezki
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