Re: [PATCH RFC] mm/vmap: map contiguous pages in batches whenever possible

[David Hildenbrand (Red Hat)]

On 11/22/25 10:03, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> In many cases, the pages passed to vmap() may include
> high-order pages—for example, the systemheap often allocates
> pages in descending order: order 8, then 4, then 0. Currently,
> vmap() iterates over every page individually—even the 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.
>
> Another possibility is to implement a new API, vmap_sg().
> However, that change seems to be quite large in scope.
>
> When vmapping a 128MB dma-buf using the systemheap,
> this RFC appears to make system_heap_do_vmap() 16× faster:
>
> W/ patch:
> [   51.363682] system_heap_do_vmap took 2474000 ns
> [   53.307044] system_heap_do_vmap took 2469008 ns
> [   55.061985] system_heap_do_vmap took 2519008 ns
> [   56.653810] system_heap_do_vmap took 2674000 ns
>
> W/o patch:
> [    8.260880] system_heap_do_vmap took 39490000 ns
> [   32.513292] system_heap_do_vmap took 38784000 ns
> [   82.673374] system_heap_do_vmap took 40711008 ns
> [   84.579062] system_heap_do_vmap took 40236000 ns
>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: John Stultz <jstultz@xxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>   mm/vmalloc.c | 49 +++++++++++++++++++++++++++++++++++++++++++------
>   1 file changed, 43 insertions(+), 6 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 0832f944544c..af2e3e8c052a 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -642,6 +642,34 @@ 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,
> +		int max_steps,
> +		unsigned int idx)

These fit into less lines.

ideally

\t\tunsigned int stride, int max_steps, unsigned int idx)

> +{

int order, nr_pages, i;
struct page *base;

But I think you can just drop "base". And order.

> +	/*
> +	 * Currently, batching is only supported in vmap_pages_range
> +	 * when page_shift == PAGE_SHIFT.
> +	 */
> +	if (stride != 1)
> +		return 0;
> +
> +	struct page *base = pages[idx];
> +	if (!PageHead(base))
> +		return 0;
> +
> +	int order = compound_order(base);
> +	int nr_pages = 1 << order;


You can drop the head check etc and simply do

nr_pages = compound_nr(pages[idx]);
if (nr_pages == 1)
	return 0;

Which raises the question: are these things folios? I assume not.

> +
> +	if (max_steps < nr_pages)
> +		return 0;
> +
> +	for (int i = 0; i < nr_pages; i++)
> +		if (pages[idx + i] != base + i)
> +			return 0;

if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
	return compound_order(pages[idx]);
return 0;

--
Cheers

David