Re: [PATCH 06/16] mm: introduce the f_op->mmap_complete, mmap_abort hooks

Lorenzo Stoakes
2025-09-09T09:38:04Z

On Tue, Sep 09, 2025 at 11:26:21AM +0200, David Hildenbrand wrote:
> > >
> > > In particular, the mmap_complete() looks like another candidate for letting
> > > a driver just go crazy on the vma? :)
> >
> > Well there's only so much we can do. In an ideal world we'd treat VMAs as
> > entirely internal data structures and pass some sort of opaque thing around, but
> > we have to keep things real here :)
>
> Right, we'd pass something around that cannot be easily abused (like
> modifying random vma flags in mmap_complete).
>
> So I was wondering if most operations that driver would perform during the
> mmap_complete() could be be abstracted, and only those then be called with
> whatever opaque thing we return here.

Well there's 2 issues at play:

1. I might end up having to rewrite _large parts_ of kernel functionality all of
which relies on there being a vma parameter (or might find that to be
intractable).

2. There's always the 'odd ones out' :) so there'll be some drivers that
absolutely do need to have access to this.

But as I was writing this I thought of an idea - why don't we have something
opaque like this, perhaps with accessor functions, but then _give the ability to
get the VMA if you REALLY have to_.

That way we can handle both problems without too much trouble.

Also Jason suggested generic functions that can just be assigned to
.mmap_complete for instance, which would obviously eliminate the crazy
factor a lot too.

I'm going to refactor to try to put ONLY prepopulate logic in
.mmap_complete where possible which fits with all of this.

>
> But I have no feeling about what crazy things a driver might do. Just
> calling remap_pfn_range() would be easy, for example, and we could abstract
> that.

Yeah, I've obviously already added some wrappers for these.

BTW I really really hate that STUPID ->vm_pgoff hack, if not for that, life
would be much simpler.

But instead now we need to specify PFN in the damn remap prepare wrapper in
case of CoW. God.

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo