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

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Message-ID: <20250915124259.GF1024672@nvidia.com>
Date: Mon, 15 Sep 2025 09:42:59 -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 Mon, Sep 15, 2025 at 01:23:30PM +0100, Lorenzo Stoakes wrote:
> On Mon, Sep 15, 2025 at 09:11:12AM -0300, Jason Gunthorpe wrote:
> > 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.
> 
> OK.
> 
> >
> > 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 need to abstract desc from action so custom handlers can perform
> sub-actions. It's unfortunate but there we go.

Why? I don't see this as required

Just mark the functions as manipulating the action using the 'action'
in the fuction name.

> > 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)
> 
> See above re: desc vs. action.

Yet, this is the API most places actually want.
 
> It'd be hard to know how to get the context right that'd need to be supplied to
> the callback.
> 
> In kcov's case it'd be kcov->area + an offset.

Just use pgoff
 
> So we'd need an offset parameter, the struct file *, whatever else to be
> passed.

Yes
 
> And then we'll find a driver where that doesn't work and we're screwed.

Bah, you keep saying that but we also may never even find one.

Jason