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

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Oct 14, 2025 at 11:27:54AM -0700, 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    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
> 
> 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.
> 
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> ---
>  mm/vmalloc.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> 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);
>
If large_order is > MAX_ORDER - 1 then there is no need even try
larger_order attempt.

>> unsigned int large_order = ilog2(nr_pages - nr_allocated);
I think, it is better to introduce "remaining" variable which
is nr_pages - nr_allocated. And on entry "remaining" can be set
to just nr_pages because "nr_allocated" is zero.

Maybe it is worth to drop/warn if __GFP_COMP is set also?

> +
> +	/*
> +	 * 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;
> +		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;
> +
> +		split_page(page, large_order);
> +		for (i = 0; i < (1U << large_order); i++)
> +			pages[nr_allocated + i] = page + i;
> +
> +		nr_allocated += 1U << large_order;
> +		large_order = ilog2(nr_pages - nr_allocated);
> +	}
>  
So this is a third path for page allocation. The question is should we
try all orders? Like already noted by Matthew, if there is no 5-order
page but there is 4-order page? Try until we check all orders. For
example we can get different order pages to fulfill the request.

The concern is then if it is a waste of high-order pages. Because we can
easily go with a single page allocator. Whereas someone in a system can not.

Apart of that, maybe we can drop the bulk_path instead of having three paths?

--
Uladzislau Rezki