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

[David Hildenbrand]

On 09.05.25 12:57, Lorenzo Stoakes wrote:
> On Fri, May 09, 2025 at 12:51:14PM +0200, David Hildenbrand wrote:
> > > > > +
> > > > > +static inline int __call_mmap_prepare(struct file *file,
> > > > > +		struct vm_area_desc *desc)
> > > > > +{
> > > > > +	return file->f_op->mmap_prepare(desc);
> > > > > +}
> > > >
> > > > Hm, is there a way avoid a copy of the exact same code from fs.h, and
> > > > essentially test the implementation in fs.h (-> more coverage by using less
> > > > duplciated stubs?).
> > >
> > > Not really, this kind of copying is sadly part of it because we're
> > > intentionally isolating vma.c from everything else, and if we try to bring
> > > in other headers they import yet others and etc. etc. it becomes a
> > > combinatorial explosion potentially.
> >
> > I guess what would work is inlining __call_mmap_prepare() -- again, rather
> > simple wrapper ... and having file_has_valid_mmap_hooks() + call_mmap()
> > reside in vma.c. Hm.
> >
> > As an alternative, we'd really need some separate header that does not allow
> > for any other includes, and is essentially only included in the other header
> > files.
> >
> > Duplicating functions in such a way that they can easily go out of sync and
> > are not getting tested is really suboptimal. :(
>
> This is a problem that already exists, if minimised. Perfect is the enemy of
> good - if we had make these tests existence depend on being able to isolate
> _everything_ they'd never happen :)
>
> But I will definitely try to improve the situation, as I couldn't agree more
> about de-syncing and it's a concern I share with you.
>
> I think we have a bit of a mess of header files anyway like this, random helpers
> put in random places etc.
>
> It doesn't help that a random driver/shm reference call_mmap()...

Yes ...

> Anyway, this is somehwat out of scope for this series, as we already have a
> number of instances like this and this is just symptomatic of an existing
> problem rather than introducing it.
>
> I think one thing to do might be to have a separate header which is explicitly
> for functions like these to at least absolutely highlight this case.

Yes, and then just include it in the relevant header files.

> The VMA tests need restructuring anyway, so it can be part of a bigger project
> to do some work cleaning up there.

Cool!

--
Cheers,

David / dhildenb