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

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Message-ID: <e7c8478b-1ce4-4a15-a185-de9d9121438c@kernel.org>
Date: Thu, 18 Dec 2025 14:01:56 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: 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>,
 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 12/15/25 06:30, 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
> 

That's quite a speedup.

> 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;

unsigned int, maybe

Why are you initializing nr_pages when you overwrite it below?

> +
> +	/*
> +	 * Currently, batching is only supported in vmap_pages_range
> +	 * when page_shift == PAGE_SHIFT.

I don't know the code so realizing how we go from page_shift to stride 
too me a second. Maybe only talk about stride here?

OTOH, is "stride" really the right terminology?

we calculate it as

	stride = 1U << (page_shift - PAGE_SHIFT);

page_shift - PAGE_SHIFT should give us an "order". So is this a 
"granularity" in nr_pages?

Again, I don't know this code, so sorry for the question.

> +	 */
> +	if (stride != 1)
> +		return 0;
> +
> +	nr_pages = compound_nr(pages[idx]);
> +	if (nr_pages == 1)
> +		return 0;
> +	if (max_steps < nr_pages)
> +		return 0;

Might combine these simple checks

if (nr_pages == 1 || max_steps < nr_pages)
	return 0;


-- 
Cheers

David