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

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Message-ID: <CAG48ez3n5AbKDC1dc7a9G0H9iHCZpc8mcR7Epuz78RB79=LaVA@mail.gmail.com>
Date: Wed, 27 Nov 2024 17:19:08 +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 Wed, Nov 27, 2024 at 1:38 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
> On Tue, Nov 26, 2024 at 10:30 PM Jann Horn <jannh@...gle.com> wrote:
> > 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...
>
> 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.