Re: [PATCH v9 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

[Jann Horn <jannh@google.com>]

On Wed, Nov 27, 2024 at 1:38 PM Alice Ryhl <aliceryhl@google.com> wrote:
> On Tue, Nov 26, 2024 at 10:30 PM Jann Horn <jannh@google.com> wrote:
> > On Fri, Nov 22, 2024 at 4:41 PM Alice Ryhl <aliceryhl@google.com> wrote:
> > > This type will be used when setting up a new vma in an f_ops->mmap()
> > > hook. Using a separate type from VmAreaRef allows us to have a separate
> > > set of operations that you are only able to use during the mmap() hook.
> > > For example, the VM_MIXEDMAP flag must not be changed after the initial
> > > setup that happens during the f_ops->mmap() hook.
> > >
> > > To avoid setting invalid flag values, the methods for clearing
> > > VM_MAYWRITE and similar involve a check of VM_WRITE, and return an error
> > > if VM_WRITE is set. Trying to use `try_clear_maywrite` without checking
> > > the return value results in a compilation error because the `Result`
> > > type is marked #[must_use].
> > >
> > > For now, there's only a method for VM_MIXEDMAP and not VM_PFNMAP. When
> > > we add a VM_PFNMAP method, we will need some way to prevent you from
> > > setting both VM_MIXEDMAP and VM_PFNMAP on the same vma.
> > >
> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> >
> > Thanks, this looks really neat!
> >
> > Reviewed-by: Jann Horn <jannh@google.com>
> >
> > > +    /// Set the `VM_IO` flag on this vma.
> > > +    ///
> > > +    /// This marks the vma as being a memory-mapped I/O region.
> >
> > nit: VM_IO isn't really exclusively used for MMIO; the header comment
> > says "Memory mapped I/O or similar", while the comment in
> > remap_pfn_range_internal() says "VM_IO tells people not to look at
> > these pages (accesses can have side effects)". But I don't really have
> > a good definition of what VM_IO actually means; so I don't really have
> > a concrete suggestion for what do do here. So my comment isn't very
> > actionable, I guess it's fine to leave this as-is unless someone
> > actually has a good definition...
>
> I can use this comment?
>
> This is used for memory mapped IO and similar. The flag tells other
> parts of the kernel to not look at the pages. For memory mapped IO
> this is useful as accesses to the pages could have side effects.

Yeah, sounds reasonable.