linux-kernel - Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f

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Message-ID: <87a5c0bdr0.fsf@kernel.org>
Date: Thu, 09 Jan 2025 09:19:47 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
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>,  "Jann Horn"
 <jannh@...gle.com>,  "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>,
  "Trevor Gross" <tmgross@...ch.edu>,  <linux-kernel@...r.kernel.org>,
  <linux-mm@...ck.org>,  <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v11 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

"Alice Ryhl" <aliceryhl@...gle.com> writes:

> On Mon, Dec 16, 2024 at 3:51 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@...gle.com> writes:
>>
>> > 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.
>> >
>> > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)
>> > Reviewed-by: Jann Horn <jannh@...gle.com>
>> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>> > ---
>> >  rust/kernel/mm/virt.rs | 181 ++++++++++++++++++++++++++++++++++++++++++++++++-
>> >  1 file changed, 180 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
>> > index 3a23854e14f4..6d9ba56d4f95 100644
>> > --- a/rust/kernel/mm/virt.rs
>> > +++ b/rust/kernel/mm/virt.rs
>> > @@ -6,7 +6,7 @@
>> >
>> >  use crate::{
>> >      bindings,
>> > -    error::{to_result, Result},
>> > +    error::{code::EINVAL, to_result, Result},
>> >      mm::MmWithUser,
>> >      page::Page,
>> >      types::Opaque,
>> > @@ -171,6 +171,185 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>> >      }
>> >  }
>> >
>> > +/// A builder for setting up a vma in an `f_ops->mmap()` hook.
>>
>> Reading this line, I would expect to be able to chain update methods as
>> in `Builder::new().prop_a().prop_b().build()`. Could/should this type
>> accommodate a proper builder pattern? Or is "builder" not the right word
>> to use here?
>
> You cannot create values of this type yourself. Only the C
> infrastructure can do so.
>
> What would you call it if not "builder"?

It looks more like a newtype with a bunch of setters and getters. It
also does not have a method to instantiate (`build()` or similar). So
how about newtype?

Best regards,
Andreas Hindborg