Re: [PATCH v6 5/6] mm/vmalloc: map contiguous pages in batches for vmap() if possible

[Dev Jain <dev.jain@arm.com>]

On 09/07/26 1:08 pm, Wen Jiang wrote:
> From: "Barry Song (Xiaomi)" <baohua@kernel.org>
> 
> 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 pages inside a high-order block are handled
> one by one.
> 
> This patch detects physically contiguous pages (regardless of whether
> they are compound or non-compound) by scanning with
> num_pages_contiguous(), and maps them as a single contiguous block
> whenever possible. The mapping order is determined by taking the
> minimum of the contiguous page count and the pfn alignment, allowing
> graceful degradation when pfn alignment is less than the contiguous
> range.
> 
> Pages with the same page_shift are coalesced and mapped via
> vmap_pages_range_noflush_walk() to avoid page table rewalk.
> 
> As users typically allocate memory in descending orders (e.g.
> 8 → 4 → 0), once an order-0 page is encountered, we stop scanning
> for contiguous pages since subsequent pages are likely order-0 as well.
> 
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> Co-developed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Wen Jiang <jiangwen6@xiaomi.com>
> Tested-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
> Tested-by: Leo Yan <leo.yan@arm.com>
> ---
>  mm/vmalloc.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 85 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d2a4d649af549..db0492151ad08 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3543,6 +3543,89 @@ void vunmap(const void *addr)
>  }
>  EXPORT_SYMBOL(vunmap);
>  
> +static inline unsigned int vm_shift(pgprot_t prot, unsigned long size)
> +{
> +	if (arch_vmap_pmd_supported(prot) && size >= PMD_SIZE)
> +		return PMD_SHIFT;
> +
> +	return arch_vmap_pte_supported_shift(size);
> +}

Need to throw in a preparatory patch for this, which will (in addition to
introduction of the vm_shift() function) do:

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index afaa14ebf17bb..dac87e1cd484b 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4175,10 +4175,7 @@ void *__vmalloc_node_range_noprof(unsigned long size, unsigned long align,
 		 * supporting them.
 		 */

-		if (arch_vmap_pmd_supported(prot) && size >= PMD_SIZE)
-			shift = PMD_SHIFT;
-		else
-			shift = arch_vmap_pte_supported_shift(size);
+		shift = vm_shift(prot, size);

 		align = max(original_align, 1UL << shift);
 	}



> +
> +static inline int get_vmap_batch_order(struct page **pages,
> +		pgprot_t prot, unsigned int max_steps, unsigned int idx)
> +{
> +	unsigned int nr_contig;
> +	int order;
> +
> +	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP))
> +		return 0;
> +
> +	nr_contig = num_pages_contiguous(&pages[idx], max_steps);
> +	if (nr_contig < 2)
> +		return 0;
> +
> +	order = ilog2(nr_contig);
> +
> +	/* Limit order by pfn alignment */
> +	order = min_t(int, order, __ffs(page_to_pfn(pages[idx])));

You missed Sashiko's point here :) the pfn may be zero and this
will blow up. You can just first derive the pfn and do the clamping
only when pfn > 0.



> +
> +	if (vm_shift(prot, PAGE_SIZE << order) == PAGE_SHIFT)
> +		return 0;
> +
> +	return order;
> +}
> +