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

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Message-ID: <aTrwN_C0JI7BuKeV@milan>
Date: Thu, 11 Dec 2025 17:24:23 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Dev Jain <dev.jain@....com>
Cc: Uladzislau Rezki <urezki@...il.com>,
	Ryan Roberts <ryan.roberts@....com>,
	"Vishal Moola (Oracle)" <vishal.moola@...il.com>,
	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 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?

--
Uladzislau Rezki