[PATCH v2 16/16] kcov: update kcov to use mmap_prepare

[Chris Mason]

On Wed, 10 Sep 2025 21:22:11 +0100 Lorenzo Stoakes <lorenzo.stoakes at oracle.com> wrote:

> We can use the mmap insert pages functionality provided for use in
> mmap_prepare to insert the kcov pages as required.
> 
> This does necessitate an allocation, but since it's in the mmap path this
> doesn't seem egregious. The allocation/freeing of the pages array is
> handled automatically by vma_desc_set_mixedmap_pages() and the mapping
> logic.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> ---
>  kernel/kcov.c | 42 ++++++++++++++++++++++++++----------------
>  1 file changed, 26 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 1d85597057e1..2bcf403e5f6f 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -484,31 +484,41 @@ void kcov_task_exit(struct task_struct *t)
>  	kcov_put(kcov);
>  }
>  
> -static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
> +static int kcov_mmap_error(int err)
> +{
> +	pr_warn_once("kcov: vm_insert_page() failed\n");
> +	return err;
> +}
> +
> +static int kcov_mmap_prepare(struct vm_area_desc *desc)
>  {
>  	int res = 0;
> -	struct kcov *kcov = vma->vm_file->private_data;
> -	unsigned long size, off;
> -	struct page *page;
> +	struct kcov *kcov = desc->file->private_data;
> +	unsigned long size, nr_pages, i;
> +	struct page **pages;
>  	unsigned long flags;
>  
>  	spin_lock_irqsave(&kcov->lock, flags);
>  	size = kcov->size * sizeof(unsigned long);
> -	if (kcov->area == NULL || vma->vm_pgoff != 0 ||
> -	    vma->vm_end - vma->vm_start != size) {
> +	if (kcov->area == NULL || desc->pgoff != 0 ||
> +	    vma_desc_size(desc) != size) {
>  		res = -EINVAL;
>  		goto exit;
>  	}
>  	spin_unlock_irqrestore(&kcov->lock, flags);
> -	vm_flags_set(vma, VM_DONTEXPAND);
> -	for (off = 0; off < size; off += PAGE_SIZE) {
> -		page = vmalloc_to_page(kcov->area + off);
> -		res = vm_insert_page(vma, vma->vm_start + off, page);
> -		if (res) {
> -			pr_warn_once("kcov: vm_insert_page() failed\n");
> -			return res;
> -		}
> -	}
> +
> +	desc->vm_flags |= VM_DONTEXPAND;
> +	nr_pages = size >> PAGE_SHIFT;
> +
> +	pages = mmap_action_mixedmap_pages(&desc->action, desc->start,
> +					   nr_pages);

Hi Lorenzo,

Not sure if it belongs here before the EINVAL tests, but it looks like
kcov->size doesn't have any page alignment.  I think size could be
4000 bytes other unaligned values, so nr_pages should round up.

-chris