linux-kernel - Re: [PATCH v9 6/8] mm: rust: add VmAreaNew for f

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Message-ID: <CAG48ez0V5B0ycPuP7eRA-xE88ks8cr+a1MFZC5emv_eAsybNAw@mail.gmail.com>
Date: Tue, 26 Nov 2024 22:29:27 +0100
From: Jann Horn <jannh@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	John Hubbard <jhubbard@...dia.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	rust-for-linux@...r.kernel.org, Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v9 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

On Fri, Nov 22, 2024 at 4:41 PM Alice Ryhl <aliceryhl@...gle.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@...gle.com>

Thanks, this looks really neat!

Reviewed-by: Jann Horn <jannh@...gle.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) };
> +    }