Re: [PATCH v2 2/3] vmalloc: Optimize vfree

[David Hildenbrand (Arm)]

On 3/16/26 16:49, Vlastimil Babka wrote:
> On 3/16/26 12:31, Muhammad Usama Anjum wrote:
>> From: Ryan Roberts <ryan.roberts@xxxxxxx>
>>
>> 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; use 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@xxxxxxx/
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Co-developed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>> ---
>> Changes since v1:
>> - Rebase on mm-new
>> - Rerun benchmarks
>> ---
>>  mm/vmalloc.c | 34 +++++++++++++++++++++++++---------
>>  1 file changed, 25 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index c607307c657a6..8b935395fb068 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -3459,18 +3459,34 @@ 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];
>> +
>> +	if (vm->nr_pages) {
>> +		bool account = !(vm->flags & VM_MAP_PUT_PAGES);
>> +		unsigned long start_pfn, pfn;
>> +		struct page *page = vm->pages[0];
>> +		int nr = 1;
>>  
>>  		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))
>> +		start_pfn = page_to_pfn(page);
>> +		if (account)
>>  			mod_lruvec_page_state(page, NR_VMALLOC, -1);
>> -		__free_page(page);
>> -		cond_resched();
>> +
>> +		for (i = 1; i < vm->nr_pages; i++) {
>> +			page = vm->pages[i];
>> +			BUG_ON(!page);
> 
> We shouldn't be adding BUG_ON()'s. Rather demote also the pre-existing one
> to VM_WARN_ON_ONCE() and skip gracefully.
> 
>> +			if (account)
>> +				mod_lruvec_page_state(page, NR_VMALLOC, -1);
> 
> I think we should be able to batch this too to use "nr"?

Are we sure that pages cannot cross nodes etc? It could happen that we
have a contig range that spans zones/nodes/etc ...

Anyhow, should we try to decouple both things, providing a
core-mm function to do the page freeing?

We do have something similar, optimized unpinning of large folios,
in unpin_user_pages_dirty_lock(). This here is a bit different.


So what I am thinking about for this code here to do:

	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);


We could optimize the first loop to do batching where possible as well.


free_pages_bulk() would match alloc_pages_bulk()

	void free_pages_bulk(struct page **page_array, unsigned long nr_pages)

Internally we'd do the contig handling.

Was that already discussed?

-- 
Cheers,

David