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

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Message-ID: <aPkZXbUON37M59As@milan>
Date: Wed, 22 Oct 2025 19:50:21 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Uladzislau Rezki <urezki@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/vmalloc: request large order pages from buddy
 allocator

On Tue, Oct 21, 2025 at 12:44:56PM -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    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
> 
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> 
> -----
> RFC:
> https://lore.kernel.org/linux-mm/20251014182754.4329-1-vishal.moola@gmail.com/
> 
> Changes since rfc:
>   - Mask off NO_FAIL in large_gfp
>   - Mask off GFP_COMP in large_gfp
> There was discussion about warning on and rejecting unsupported GFP
> flags in vmalloc, I'll have a separate patch for that.
> 
>   - Introduce nr_remaining variable to track total pages
>   - Calculate large order as (min(max_order, ilog2())
>   - Attempt lower orders on failure before falling back to original path
>   - Drop unnecessary fallback comment change
> ---
>  mm/vmalloc.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index adde450ddf5e..0832f944544c 100644
> --- 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;
> +	unsigned int large_order = ilog2(nr_remaining);
> +
> +	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
> 
I like the idea of page allocation using larger-order :)

Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>

--
Uladzislau Rezki