linux-kernel - Re: [PATCH v2 16/16] kcov: update kcov to use mmap

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Message-ID: <17bef9e0-575f-4ced-9884-3fd5a8f77067@lucifer.local>
Date: Fri, 19 Sep 2025 06:10:12 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Chris Mason <clm@...a.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, Guo Ren <guoren@...nel.org>,
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Subject: Re: [PATCH v2 16/16] kcov: update kcov to use mmap_prepare

On Thu, Sep 18, 2025 at 12:45:38PM -0700, Chris Mason wrote:
> On Wed, 10 Sep 2025 21:22:11 +0100 Lorenzo Stoakes <lorenzo.stoakes@...cle.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@...cle.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.

Thanks, you may well be right, but but this series has been respun and I no
longer touch kcov. :)

Am at v4 now -
https://lore.kernel.org/linux-mm/cover.1758135681.git.lorenzo.stoakes@oracle.com/
- apologies for the quick turnaround but going to kernel recipes soon and then
on vacation so wanted to get this wrapped up!

>
> -chris

Cheers, Lorenzo