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

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Message-ID: <872a06d7-6b74-410c-a0fe-0a64ae1efd9b@lucifer.local>
Date: Mon, 15 Sep 2025 13:43:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v2 16/16] kcov: update kcov to use mmap_prepare

On Mon, Sep 15, 2025 at 09:16:17AM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 10, 2025 at 09:22:11PM +0100, Lorenzo Stoakes wrote:
> > +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) {
>
> IMHO these range checks should be cleaned up into a helper:
>
> /* Returns true if the VMA falls within starting_pgoff to
>      starting_pgoff + ROUND_DOWN(length_bytes, PAGE_SIZE))
>    Is careful to avoid any arithmetic overflow.
>  */

Right, but I can't refactor every driver I touch, it's not really tractable. I'd
like to get this change done before I retire :)

> vma_desc_check_range(desc, starting_pgoff=0, length_bytes=size);
>
> > +	desc->vm_flags |= VM_DONTEXPAND;
> > +	nr_pages = size >> PAGE_SHIFT;
> > +
> > +	pages = mmap_action_mixedmap_pages(&desc->action, desc->start,
> > +					   nr_pages);
> > +	if (!pages)
> > +		return -ENOMEM;
> > +
> > +	for (i = 0; i < nr_pages; i++)
> > +		pages[i] = vmalloc_to_page(kcov->area + i * PAGE_SIZE);
>
> This is not a mixed map.
>
> All the memory comes from vmalloc_user() which makes them normal
> struct pages with refcounts.
>
> If anything the action should be called mmap_action_vmalloc_user() to
> match how the memory was allocated instead of open coding something.

Again we're getting into the same issue - my workload doesn't really permit
me to refactor every user of .mmap beyond converting sensibly to the new
scheme.

I think this kind of change is out of scope for the series.

I'd rather make this as apples-to-apples as possible for now so it can be
done vaguely mechanically.

Of course we can follow up with improvements later.

>
> Jason

Cheers, Lorenzo