Re: [PATCH v2 1/3] mm: introduce new .mmap_prepare() file callback

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, May 14, 2025 at 11:01:49AM +0100, Pedro Falcato wrote:
> On Wed, May 14, 2025 at 10:12:49AM +0100, Lorenzo Stoakes wrote:
> > On Wed, May 14, 2025 at 10:04:06AM +0100, Pedro Falcato wrote:
> > > On Fri, May 09, 2025 at 01:13:34PM +0100, Lorenzo Stoakes wrote:
> > > > Provide a means by which drivers can specify which fields of those
> > > > permitted to be changed should be altered to prior to mmap()'ing a
> > > > range (which may either result from a merge or from mapping an entirely new
> > > > VMA).
> > > >
> > > > Doing so is substantially safer than the existing .mmap() calback which
> > > > provides unrestricted access to the part-constructed VMA and permits
> > > > drivers and file systems to do 'creative' things which makes it hard to
> > > > reason about the state of the VMA after the function returns.
> > > >
> > > > The existing .mmap() callback's freedom has caused a great deal of issues,
> > > > especially in error handling, as unwinding the mmap() state has proven to
> > > > be non-trivial and caused significant issues in the past, for instance
> > > > those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> > > > error path behaviour").
> > > >
> > > > It also necessitates a second attempt at merge once the .mmap() callback
> > > > has completed, which has caused issues in the past, is awkward, adds
> > > > overhead and is difficult to reason about.
> > > >
> > > > The .mmap_prepare() callback eliminates this requirement, as we can update
> > > > fields prior to even attempting the first merge. It is safer, as we heavily
> > > > restrict what can actually be modified, and being invoked very early in the
> > > > mmap() process, error handling can be performed safely with very little
> > > > unwinding of state required.
> > > >
> > > > The .mmap_prepare() and deprecated .mmap() callbacks are mutually
> > > > exclusive, so we permit only one to be invoked at a time.
> > > >
> > > > Update vma userland test stubs to account for changes.
> > > >
> > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > >
> > > Reviewed-by: Pedro Falcato <pfalcato@suse.de>
> > >
> > > Neat idea, thanks. This should also help out with the insane proliferation of
> > > vm_flags_set in ->mmap() callbacks all over. Hopefully.
> [snip]
> > >
> > > 2) Possibly add a ->mmap_finish()? With a fully constructed vma at that point.
> > >    So things like remap_pfn_range can still be used by drivers' mmap()
> > >    implementation.
> >
> > Thanks for raising the remap_pfn_range() case! Yes this is definitely a
> > thing.
> >
> > However this proposed callback would totally undermine the purpose of the
> > change - the idea is to never give a vma because if we do so we lose all of
> > the advantages here and may as well just leave the mmap in place for
> > this...
> >
>
> Yes, good point.
>
> > However I do think we'll need a new callback at some point (previously
> > discussed in thread).
> >
> > We could perhaps provide the option to _explicitly_ remap for instance. I
> > would want it to be heavily locked down as to what can happen and to happen
> > as early as possible.
> >
>
> I think we can simply combine various ideas here. Like:
>
> struct vm_area_desc_private {
> 	struct vm_area_desc desc;
> 	struct vm_area_struct *vma;
> };
>
> Then, for this "mmap_finish" callback and associated infra:
>
> 	int (*mmap_finish)(struct vm_area_desc *desc);
>
> int mmap_remap_pfn_range(struct vm_area_desc *desc, /*...*/)
> {
> 	struct vm_area_desc_private *private = container_of(desc, struct vm_area_desc_private, desc);
> 	return remap_pfn_range(private->vma, /*...*/);
> }
>
> int random_driver_mmap_finish(struct vm_area_desc *desc)
> {
> 	return mmap_remap_pfn_range(desc, desc->start, some_pfn, some_size,
> 				    desc->page_prot);
> }
>
>

Yeah that's really nice actually! I like that. Will note down for when we
come to this.

Obviously we need to know whent the finish implies a remap, but we can have
a field for this potentially set in the vm_area_desc to indicate this.

> I think something of the sort would be quite less prone to abuse, and we could
> take the time to then even polish up the interface (e.g maybe it would be nicer
> if mmap_remap_pfn_range took a vma offset, and not a start address).

Right yeah, David definitely wanted some improvements in this area so this
aligns with other intentions/work.

>
> Anyway, just brainstorming. This idea came to mind, I think it's quite interesting.

Nice ideas :) yeah, good chance to undo some sins here.

>
> > This is something we can iterate on, as trying to find the ideal scheme
> > immediately will just lead to inaction, the big advantage with the approach
> > here is we can be iterative.
> >
> > We provide this, use it in a scenario which allows us to eliminate merge
> > retry, and can take it from there :)
>
> Yep, totally.
>
> >
> > So indeed, watch this space basically... I will be highly proactive on this
> > stuff moving forward.
> >
> > >
> > > 1) is particularly important so our VFS and driver friends know this is supposed
> > > to be The Way Forward.
> >
> > I think probably the answer is for me to fully update the document to be
> > bang up to date, right? But that would obviously work best as a follow up
> > patch :)
> >
>
> You love your big projects :p I had the impression the docs were more or less up to date?
> The VFS people do update it somewhat diligently. And for mm we only have ->mmap, ->get_unmapped_area,
> and now ->mmap_prepare. And the descriptions are ATM quite useless, just
> "called by the mmap(2) system call".

Much like leg day, sometimes painful things are worthwhile things :)

As for docs I saw there were some missing uring entries, and it explicitly
says 'as of kernel 4.18'.

So I feel that it really needs to be properly updated and it ends up being
a little out of scope to the series.

Since this series only uses this callback in one, mm-specific, place, I
think it makes sense to attack this on a follow up, alongside updating file
system callbacks etc.

But it's definitely on the todo! :)

Cheers, Lorenzo

>
> --
> Pedro