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

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

On Wed, Oct 15, 2025 at 04:56:42AM +0100, Matthew Wilcox wrote:
> On Tue, Oct 14, 2025 at 11:27:54AM -0700, Vishal Moola (Oracle) wrote:
> > Running 1000 iterations of allocations on a small 4GB system finds:
> > 
> > 1000 2mb allocations:
> > 	[Baseline]			[This patch]
> > 	real    46.310s			real    34.380s
> > 	user    0.001s			user    0.008s
> > 	sys     46.058s			sys     34.152s
> > 
> > 10000 200kb allocations:
> > 	[Baseline]			[This patch]
> > 	real    56.104s			real    43.946s
> > 	user    0.001s			user    0.003s
> > 	sys     55.375s			sys     43.259s
> > 
> > 10000 20kb allocations:
> > 	[Baseline]			[This patch]
> > 	real    0m8.438s		real    0m9.160s
> > 	user    0m0.001s		user    0m0.002s
> > 	sys     0m7.936s		sys     0m8.671s
> 
> I'd be more confident in the 20kB numbers if you'd done 10x more
> iterations.

I actually ran my a number of times to mitigate the effects of possibly
too small sample sizes, so I do have that number for you too:

[Baseline]			[This patch]
real    1m28.119s		real    1m32.630s
user    0m0.012s		user    0m0.011s
sys     1m23.270s		sys     1m28.529s

> Also, I think 20kB is probably an _interesting_ number, but it's not
> going to display your change to its best advantage.  A 32kB allocation
> will look much better, for example.

I provided those particular numbers to showcase the beneficial cases as
well as the regression case.

I ended up finding that allocating sizes <=20k had noticeable
regressions, while [20k, 90k] was approximately the same, and >= 90k had
improvements (getting more and more noticeable as size grows in
magnitude).

> Also, can you go into more detail of the test?  Based on our off-list
> conversation, we were talking about allocating something like 100MB
> of memory (in these various sizes) then freeing it, just to be sure
> that we're measuring the performance of the buddy allocator and
> not the PCP list.

Yup.

What I did to get the numbers above was: call vmalloc() n number of
times on that particular size, then free all those allocations. Then,
I did 1000 iterations of that to get a better average.

So none of these allocations were freed until all the allocations were
done, every single time.

> > This is an RFC, comments and thoughts are welcomed. There is a
> > clear benefit to be had for large allocations, but there is
> > some regression for smaller allocations.
> 
> Also we think that there's probably a later win to be had by
> not splitting the page we allocated.
> 
> At some point, we should also start allocating frozen pages
> for vmalloc.  That's going to be interesting for the users which
> map vmalloc pages to userspace.
> 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 97cef2cc14d3..0a25e5cf841c 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3621,6 +3621,38 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
> >  	unsigned int nr_allocated = 0;
> >  	struct page *page;
> >  	int i;
> > +	gfp_t large_gfp = (gfp & ~__GFP_DIRECT_RECLAIM) | __GFP_NOWARN;
> > +	unsigned int large_order = ilog2(nr_pages - nr_allocated);
> > +
> > +	/*
> > +	 * Initially, attempt to have the page allocator give us large order
> > +	 * pages. Do not attempt allocating smaller than order chunks since
> > +	 * __vmap_pages_range() expects physically contigous pages of exactly
> > +	 * order long chunks.
> > +	 */
> > +	while (large_order > order && nr_allocated < nr_pages) {
> > +		/*
> > +		 * High-order nofail allocations are really expensive and
> > +		 * potentially dangerous (pre-mature OOM, disruptive reclaim
> > +		 * and compaction etc.
> > +		 */
> > +		if (gfp & __GFP_NOFAIL)
> > +			break;
> 
> sure, but we could just clear NOFAIL from the large_gfp flags instead
> of giving up on this path so quickly?

Yeah I'll do that.

> > +		if (nid == NUMA_NO_NODE)
> > +			page = alloc_pages_noprof(large_gfp, large_order);
> > +		else
> > +			page = alloc_pages_node_noprof(nid, large_gfp, large_order);
> > +
> > +		if (unlikely(!page))
> > +			break;
> 
> I'm not entirely convinced here.  We might want to fall back to the next
> larger size.  eg if we try to allocate an order-6 page, and there's not
> one readily available, perhaps we should try to allocate an order-5 page
> instead of falling back to the bulk allocator?

I'll try that out and see how that affects the numbers.

> > @@ -3665,7 +3697,7 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
> >  		}
> >  	}
> >  
> > -	/* High-order pages or fallback path if "bulk" fails. */
> > +	/* High-order arch pages or fallback path if "bulk" fails. */
> 
> I'm not quite clear what this comment change is meant to convey?

Ah that was a comment I had inserted to remind myself that the passed in
order is tied to the HAVE_ARCH_HUGE_VMALLOC config. I meant to leave
that out.