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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 21 Oct 2025 12:44:56 -0700 "Vishal Moola (Oracle)" <vishal.moola@gmail.com> 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.

Does this have potential to inadvertently reduce the availability of
hugepages?

> 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

Nice, but how significant is this change likely to be for a real workload?

> ...
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3619,8 +3619,44 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
>  		unsigned int order, unsigned int nr_pages, struct page **pages)
>  {
>  	unsigned int nr_allocated = 0;
> +	unsigned int nr_remaining = nr_pages;
> +	unsigned int max_attempt_order = MAX_PAGE_ORDER;
>  	struct page *page;
>  	int i;
> +	gfp_t large_gfp = (gfp &
> +		~(__GFP_DIRECT_RECLAIM | __GFP_NOFAIL | __GFP_COMP))
> +		| __GFP_NOWARN;

Gee, why is this so complicated?

> +	unsigned int large_order = ilog2(nr_remaining);

Should nr_remaining be rounded up to next-power-of-two?

> +	large_order = min(max_attempt_order, large_order);
> +
> +	/*
> +	 * 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_remaining) {
> +		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)) {
> +			max_attempt_order = --large_order;
> +			continue;
> +		}
> +
> +		split_page(page, large_order);
> +		for (i = 0; i < (1U << large_order); i++)
> +			pages[nr_allocated + i] = page + i;
> +
> +		nr_allocated += 1U << large_order;
> +		nr_remaining = nr_pages - nr_allocated;
> +
> +		large_order = ilog2(nr_remaining);
> +		large_order = min(max_attempt_order, large_order);
> +	}
>  
>  	/*
>  	 * For order-0 pages we make use of bulk allocator, if
> -- 
> 2.51.0