Re: [PATCH v6 1/2] rust: mm: add abstractions for mm_struct and vm_area_struct

[Jann Horn <jannh@google.com>]

On Fri, Nov 8, 2024 at 8:01 PM Jann Horn <jannh@google.com> wrote:
> On Thu, Oct 10, 2024 at 2:56 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > These abstractions allow you to manipulate vmas. Rust Binder will uses
> > these in a few different ways.
> >
> > In the mmap implementation, a VmAreaNew will be provided to the mmap
> > call which allows it to modify the vma in ways that are only okay during
> > initial setup. This is the case where the most methods are available.
> >
> > However, Rust Binder needs to insert and remove pages from the vma as
> > time passes. When incoming messages arrive, pages may need to be
> > inserted if space is missing, and in this case that is done by using a
> > stashed ARef<Mm> and calling mmget_not_zero followed by mmap_write_lock
> > followed by vma_lookup followed by vm_insert_page. In this case, since
> > mmap_write_lock is used, the VmAreaMut type will be in use.
>
> FYI, the way the C binder implementation uses vma_lookup() and
> vm_insert_page() is not very idiomatic. The standard way of
> implementing binder_alloc_free_page() would be to use something like
> unmap_mapping_range() instead of using
> vma_lookup()+zap_page_range_single(); though to set that up, you'd
> have to create one inode per binder file, maybe with something like
> the DRM subsystem's drm_fs_inode_new(). And instead of having
> binder_install_single_page(), the standard way would be to let
> binder_vm_fault() install PTEs lazily on fault. That way you'd never
> have to take mmap locks or grab MM references yourself.

Let me know if you think it would be helpful to see a prototype of
that in C - I think I can cobble that together, but doing it nicely
will require some work to convert at least some of the binder_alloc
locking to mutexes, and some more work to switch the ->f_mapping of
the binder file or something like that. (I guess modeling that in Rust
might be a bit of a pain though...)