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

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Message-ID: <5be340e8-353a-4cde-8770-136a515f326a@lucifer.local>
Date: Mon, 15 Sep 2025 13:54:05 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
        Guo Ren <guoren@...nel.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 09:42:59AM -0300, Jason Gunthorpe wrote:
> 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.

Because now sub-callers that partially map using one method and partially map
using another now need to have a desc too that they have to 'just know' which
fields to update or artificially set up.

The vmcore case does something like this.

Instead, we have actions where it's 100% clear what's going to happen.

>
> > > 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.

OK let me try something like this, then. I guess I can update it later if
we discover such a dirver.

>
> Jason

Cheers, Lorenzo