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

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Message-ID: <aSiB-UsunuE7u295@milan>
Date: Thu, 27 Nov 2025 17:53:13 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	John Stultz <jstultz@...gle.com>,
	Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH RFC] mm/vmap: map contiguous pages in batches whenever
 possible

On Sat, Nov 22, 2025 at 05:03:43PM +0800, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> 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@...il.com>
> Cc: Sumit Semwal <sumit.semwal@...aro.org>
> Cc: John Stultz <jstultz@...gle.com>
> Cc: Maxime Ripard <mripard@...nel.org>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  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)
> +{
> +	/*
> +	 * 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;
> +
> +	if (max_steps < nr_pages)
> +		return 0;
> +
> +	for (int i = 0; i < nr_pages; i++)
> +		if (pages[idx + i] != base + i)
> +			return 0;
> +	return order;
> +}
> +
>  /*
>   * vmap_pages_range_noflush is similar to vmap_pages_range, but does not
>   * flush caches.
> @@ -655,23 +683,32 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>  		pgprot_t prot, struct page **pages, unsigned int page_shift)
>  {
>  	unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
> +	unsigned int stride;
>  
>  	WARN_ON(page_shift < PAGE_SHIFT);
>  
> +	/*
> +	 * Some users may allocate pages from high-order down to order 0.
> +	 * We roughly check if the first page is a compound page. If so,
> +	 * there is a chance to batch multiple pages together.
> +	 */
>  	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> -			page_shift == PAGE_SHIFT)
> +			(page_shift == PAGE_SHIFT && !PageCompound(pages[0])))
>
Do we support __GFP_COMP as vmalloc/vmap flag? As i see from latest:

/*
 * See __vmalloc_node_range() for a clear list of supported vmalloc flags.
 * This gfp lists all flags currently passed through vmalloc. Currently,
 * __GFP_ZERO is used by BPF and __GFP_NORETRY is used by percpu. Both drm
 * and BPF also use GFP_USER. Additionally, various users pass
 * GFP_KERNEL_ACCOUNT. Xfs uses __GFP_NOLOCKDEP.
 */
#define GFP_VMALLOC_SUPPORTED (GFP_KERNEL | GFP_ATOMIC | GFP_NOWAIT |\
                               __GFP_NOFAIL |  __GFP_ZERO | __GFP_NORETRY |\
                               GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\
                               GFP_USER | __GFP_NOLOCKDEP)

Could you please clarify when PageCompound(pages[0]) returns true?

>  		return vmap_small_pages_range_noflush(addr, end, prot, pages);
>  
> -	for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> -		int err;
> +	stride = 1U << (page_shift - PAGE_SHIFT);
> +	for (i = 0; i < nr; ) {
> +		int err, order;
>  
> -		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> +		order = get_vmap_batch_order(pages, stride, nr - i, i);
> +		err = vmap_range_noflush(addr, addr + (1UL << (page_shift + order)),
>  					page_to_phys(pages[i]), prot,
> -					page_shift);
> +					page_shift + order);
>  		if (err)
>  			return err;
>  
> -		addr += 1UL << page_shift;
> +		addr += 1UL  << (page_shift + order);
> +		i += 1U << (order + page_shift - PAGE_SHIFT);
>  	}
>  
>  	return 0;
> -- 
> 2.39.3 (Apple Git-146)
>