Re: [RFC PATCH 1/3] mm: introduce new .mmap_proto() f_op callback

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

On Thu, May 01, 2025 at 09:51:26AM -0400, Liam R. Howlett wrote:
> * David Hildenbrand <david@redhat.com> [250430 17:58]:
> > On 30.04.25 21:54, 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_proto() 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.
> > >
> > > Update vma userland test stubs to account for changes.
> > >
> > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >
> >
> > I really don't like the "proto" terminology. :)
> >
> > [yes, David and his naming :P ]
> >
> > No, the problem is that it is fairly unintuitive what is happening here.
> >
> > Coming from a different direction, the callback is trigger after
> > __mmap_prepare() ... could we call it "->mmap_prepare" or something like
> > that? (mmap_setup, whatever)
> >
> > Maybe mmap_setup and vma_setup_param? Just a thought ...
>
> Although I don't really mind what we call this, I don't like the flags
> name.  Can we qualify it with vm_flags?  It looks dumb most of the time
> but we have had variables named "flags" set to the wrong flag type make
> it through code review and into the kernel.

Sure, will do!

>
> That is, we may see people set a struct vma_proto proto later do
> proto.flags = map_flags.  It sounds stupid here, but we have had cases
> of exactly this making it through to a kernel release.
>
> I bring this up here because it may influence the prefix of the setup
> call, or vice versa... and not _just_ to derail another renaming.

;)

Yeah, 'flags' is one of the more ambigious names in the kernel
generally... I did go back and forth on this one but this is a good point,
and it's an easy mistake to make, sadly...

>
> >
> >
> > In general (although it's late in Germany), it does sound like an
> > interesting approach.
> >
> > How feasiable is it to remove ->mmap in the long run, and would we maybe
> > need other callbacks to make that possible?
> >
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >
>