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

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Message-ID: <f5bc0a98-3ab2-4dad-bf05-917528744ef7@arm.com>
Date: Thu, 11 Dec 2025 21:13:28 +0530
From: Dev Jain <dev.jain@....com>
To: Uladzislau Rezki <urezki@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/vmalloc: request large order pages from buddy
 allocator


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.

>
> Any thoughts?
>
> --
> Uladzislau Rezki
>