linux-kernel - Re: [PATCH] mm/vmalloc: map contiguous pages in batches for vmap() whenever possible

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Message-ID: <aUQJEa643lQAGK6s@milan>
Date: Thu, 18 Dec 2025 15:00:49 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	dri-devel@...ts.freedesktop.org, jstultz@...gle.com,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
	David Hildenbrand <david@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Maxime Ripard <mripard@...nel.org>,
	Tangquan Zheng <zhengtangquan@...o.com>
Subject: Re: [PATCH] mm/vmalloc: map contiguous pages in batches for vmap()
 whenever possible

On Mon, Dec 15, 2025 at 01:30:50PM +0800, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> In many cases, the pages passed to vmap() may include high-order
> pages allocated with __GFP_COMP flags. For example, the systemheap
> often allocates pages in descending order: order 8, then 4, then 0.
> Currently, vmap() iterates over every page individually—even pages
> inside a high-order block are handled one by one.
> 
> This patch detects high-order pages and maps them as a single
> contiguous block whenever possible.
> 
> An alternative would be to implement a new API, vmap_sg(), but that
> change seems to be large in scope.
> 
> When vmapping a 128MB dma-buf using the systemheap, this patch
> makes system_heap_do_vmap() roughly 17× faster.
> 
> W/ patch:
> [   10.404769] system_heap_do_vmap took 2494000 ns
> [   12.525921] system_heap_do_vmap took 2467008 ns
> [   14.517348] system_heap_do_vmap took 2471008 ns
> [   16.593406] system_heap_do_vmap took 2444000 ns
> [   19.501341] system_heap_do_vmap took 2489008 ns
> 
> W/o patch:
> [    7.413756] system_heap_do_vmap took 42626000 ns
> [    9.425610] system_heap_do_vmap took 42500992 ns
> [   11.810898] system_heap_do_vmap took 42215008 ns
> [   14.336790] system_heap_do_vmap took 42134992 ns
> [   16.373890] system_heap_do_vmap took 42750000 ns
> 
> Cc: David Hildenbrand <david@...nel.org>
> Cc: Uladzislau Rezki <urezki@...il.com>
> Cc: Sumit Semwal <sumit.semwal@...aro.org>
> Cc: John Stultz <jstultz@...gle.com>
> Cc: Maxime Ripard <mripard@...nel.org>
> Tested-by: Tangquan Zheng <zhengtangquan@...o.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  * diff with rfc:
>  Many code refinements based on David's suggestions, thanks!
>  Refine comment and changelog according to Uladzislau, thanks!
>  rfc link:
>  https://lore.kernel.org/linux-mm/20251122090343.81243-1-21cnbao@gmail.com/
> 
>  mm/vmalloc.c | 45 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 39 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 41dd01e8430c..8d577767a9e5 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -642,6 +642,29 @@ static int vmap_small_pages_range_noflush(unsigned long addr, unsigned long end,
>  	return err;
>  }
>  
> +static inline int get_vmap_batch_order(struct page **pages,
> +		unsigned int stride, unsigned int max_steps, unsigned int idx)
> +{
> +	int nr_pages = 1;
> +
> +	/*
> +	 * Currently, batching is only supported in vmap_pages_range
> +	 * when page_shift == PAGE_SHIFT.
> +	 */
> +	if (stride != 1)
> +		return 0;
> +
> +	nr_pages = compound_nr(pages[idx]);
> +	if (nr_pages == 1)
> +		return 0;
> +	if (max_steps < nr_pages)
> +		return 0;
> +
> +	if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
> +		return compound_order(pages[idx]);
> +	return 0;
> +}
> +
Can we instead look at this as: it can be that we have continues
set of pages let's find out. I mean if we do not stick just to
compound pages.

--
Uladzislau Rezki