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

Uladzislau Rezki
2025-12-18T14:00:57Z

On Mon, Dec 15, 2025 at 01:30:50PM +0800, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.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@kernel.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: John Stultz <jstultz@google.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Tested-by: Tangquan Zheng <zhengtangquan@oppo.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.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