Re: [PATCH] mm/vmalloc: request large order pages from buddy allocator

[Dev Jain <dev.jain@arm.com>]

On 11/12/25 9:54 pm, Uladzislau Rezki wrote:
> On Thu, Dec 11, 2025 at 09:13:28PM +0530, Dev Jain wrote:
>> On 11/12/25 9:09 pm, Uladzislau Rezki wrote:
>>> On Thu, Dec 11, 2025 at 03:28:56PM +0000, Ryan Roberts wrote:
>>>> On 10/12/2025 22:28, Vishal Moola (Oracle) wrote:
>>>>> On Wed, Dec 10, 2025 at 01:21:22PM +0000, Ryan Roberts wrote:
>>>>>> Hi Vishal,
>>>>>>
>>>>>>
>>>>>> On 21/10/2025 20:44, Vishal Moola (Oracle) wrote:
>>>>>>> Sometimes, vm_area_alloc_pages() will want many pages from the buddy
>>>>>>> allocator. Rather than making requests to the buddy allocator for at
>>>>>>> most 100 pages at a time, we can eagerly request large order pages a
>>>>>>> smaller number of times.
>>>>>>>
>>>>>>> We still split the large order pages down to order-0 as the rest of the
>>>>>>> vmalloc code (and some callers) depend on it. We still defer to the bulk
>>>>>>> allocator and fallback path in case of order-0 pages or failure.
>>>>>>>
>>>>>>> Running 1000 iterations of allocations on a small 4GB system finds:
>>>>>>>
>>>>>>> 1000 2mb allocations:
>>>>>>> 	[Baseline]			[This patch]
>>>>>>> 	real    46.310s			real    0m34.582
>>>>>>> 	user    0.001s			user    0.006s
>>>>>>> 	sys     46.058s			sys     0m34.365s
>>>>>>>
>>>>>>> 10000 200kb allocations:
>>>>>>> 	[Baseline]			[This patch]
>>>>>>> 	real    56.104s			real    0m43.696
>>>>>>> 	user    0.001s			user    0.003s
>>>>>>> 	sys     55.375s			sys     0m42.995s
>>>>>> I'm seeing some big vmalloc micro benchmark regressions on arm64, for which
>>>>>> bisect is pointing to this patch.
>>>>> Ulad had similar findings/concerns[1]. Tldr: The numbers you are seeing
>>>>> are expected for how the test module is currently written.
>>>> Hmm... simplistically, I'd say that either the tests are bad, in which case they
>>>> should be deleted, or they are good, in which case we shouldn't ignore the
>>>> regressions. Having tests that we learn to ignore is the worst of both worlds.
>>>>
>>> Uh.. Tests are for measure vmalloc performance and stressing. They can not be just
>>> removed :) In some sense they are synthetic, from the other hand they allow to find
>>> problems and bottle-necks + measure perf. You have identified regression with it :)
>>>
>>> I think, the problem is in the
>>>
>>> +   14.05%     0.11%  [kernel]          [k] remove_vm_area
>>> +   11.85%     1.82%  [kernel]          [k] __alloc_frozen_pages_noprof
>>> +   10.91%     0.36%  [kernel]          [k] __get_vm_area_node
>>> +   10.60%     7.58%  [kernel]          [k] insert_vmap_area
>>> +   10.02%     4.67%  [kernel]          [k] get_page_from_freelist
>>>
>>>
>>> get_page_from_freelist() call. With a patch it adds 10% of cycles on
>>> top whereas without patch i do not see the symbol at all, i.e. pages
>>> are obtained really fast from the pcp list, not from the body.
>>>
>>> The question is, why high-order pages are not end-up in the pcp-cache?
>>> I think it is due to the fact, that we split such pages and freeing them
>>> as order-0 one.
>> Please take a look at my RFC:
>>
>> https://lore.kernel.org/all/20251112110807.69958-1-dev.jain@arm.com/
>>
>> You are right, we allocate large folios but then split them up and free
>> them as basepages. In patch 2 I have proved (not rigorously) that pcp
>> draining is one of the issues.
>>
> You sent out RFC 12 of NOV :-/ I have missed those two patches from you,
> even though you put me into "to".
>
> Appreciate that you point me on your work. Let me have a look at this.
>
> Could you please resend RFC based on latest code-base?

Yup I'll do that. I was trying to get some perf numbers from LTP - fsstress,
but the variance seems to be high on the system I am testing. I would
appreciate if you or someone can run some benchmarks (filesystem is what I
believe would benefit).

>
> --
> Uladzislau Rezki
>