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

[Jann Horn <jannh@google.com>]

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...

> +    #[inline]
> +    pub fn set_io(&self) {
> +        // SAFETY: Setting the VM_IO flag is always okay.
> +        unsafe { self.update_flags(flags::IO, 0) };
> +    }