linux-kernel - Re: [PATCH v2 08/16] mm: add ability to take further action in vm_area

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Message-ID: <20250915121112.GC1024672@nvidia.com>
Date: Mon, 15 Sep 2025 09:11:12 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v2 08/16] mm: add ability to take further action in
 vm_area_desc

On Wed, Sep 10, 2025 at 09:22:03PM +0100, Lorenzo Stoakes wrote:
> +static inline void mmap_action_remap(struct mmap_action *action,
> +		unsigned long addr, unsigned long pfn, unsigned long size,
> +		pgprot_t pgprot)
> +{
> +	action->type = MMAP_REMAP_PFN;
> +
> +	action->remap.addr = addr;
> +	action->remap.pfn = pfn;
> +	action->remap.size = size;
> +	action->remap.pgprot = pgprot;
> +}

These helpers drivers are supposed to call really should have kdocs.

Especially since 'addr' is sort of ambigous.

And I'm wondering why they don't take in the vm_area_desc? Eg shouldn't
we be strongly discouraging using anything other than
vma->vm_page_prot as the last argument?

I'd probably also have a small helper wrapper for the very common case
of whole vma:

/* Fill the entire VMA with pfns starting at pfn. Caller must have 
 * already checked desc has an appropriate size */
mmap_action_remap_full(struct vm_area_desc *desc, unsigned long pfn)

It is not normal for a driver to partially populate a VMA, lets call
those out as something weird.

> +struct page **mmap_action_mixedmap_pages(struct mmap_action *action,
> +		unsigned long addr, unsigned long num_pages)
> +{
> +	struct page **pages;
> +
> +	pages = kmalloc_array(num_pages, sizeof(struct page *), GFP_KERNEL);
> +	if (!pages)
> +		return NULL;

This allocation seems like a shame, I doubt many places actually need
it .. A callback to get each pfn would be better?

Jason