linux-kernel - Re: [PATCH v8 7/7] rust: miscdevice: add mmap support

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Message-ID: <6a8b01a7-0088-4635-a542-cb1a8146ecfa@lucifer.local>
Date: Thu, 21 Nov 2024 13:11:01 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>,
        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>,
        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 v8 7/7] rust: miscdevice: add mmap support

On Thu, Nov 21, 2024 at 11:08:05AM +0100, Alice Ryhl wrote:
> On Wed, Nov 20, 2024 at 9:07 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > On Wed, Nov 20, 2024 at 02:50:01PM +0000, Alice Ryhl wrote:
> > > Using the vma support introduced by the previous commit, introduce mmap
> > > support for miscdevices. The mmap call is given a vma that is undergoing
> > > initial setup, so the VmAreaNew type is used.
> >
> > Again, I'd be explicit about the VMA being passed to a struct
> > file_operations->mmap() hook on mmap. Otherwise this seems super vague to
> > me!
>
> I wasn't sure if vmas could be constructed in any context other than
> mmap. Will reword.

Thanks.

>
> > > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

_Definitely_ need the commit msg update above, but other than this from mm side
looks reasonable, so:

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)

> > > ---
> > >  rust/kernel/miscdevice.rs | 28 ++++++++++++++++++++++++++++
> > >  1 file changed, 28 insertions(+)
> > >
> > > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > > index 7e2a79b3ae26..4e4b9476e092 100644
> > > --- a/rust/kernel/miscdevice.rs
> > > +++ b/rust/kernel/miscdevice.rs
> > > @@ -11,6 +11,7 @@
> > >  use crate::{
> > >      bindings,
> > >      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> > > +    mm::virt::VmAreaNew,
> > >      prelude::*,
> > >      str::CStr,
> > >      types::{ForeignOwnable, Opaque},
> > > @@ -110,6 +111,11 @@ fn release(device: Self::Ptr) {
> > >          drop(device);
> > >      }
> > >
> > > +    /// Handle for mmap.
> > > +    fn mmap(_device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>, _vma: &VmAreaNew) -> Result {
> >
> > You will have to help me with this :) ForeignOwnable, Borrowed<'_>, _vma I'm a
> > bit lost here!
>
> When you implement the Miscdevice for your own type, you write a block
> like this one:
>
> impl Miscdevice for MyType {
>     type Ptr = Arc<MyType>;
>     ...
> }
>
> Here Arc is a pointer type (very similar to ARef) that represents
> ownership over a refcount to a refcounted value.
>
> In this case:
> * Self refers to MyType.
> * Self::Ptr refers to Arc<MyType>
> * <Self::Ptr as ForeignOwnable>::Borrowed<'_> refers to ArcBorrow<MyType>
>
> The last step is resolved to ArcBorrow<MyType> because of this impl
> block in rust/kernel/sync/arc.rs:
>
> impl<T: 'static> ForeignOwnable for Arc<T> {
>     type Borrowed<'a> = ArcBorrow<'a, T>;
>     ...
> }
>
> Note that Self::Ptr is short-hand for <Self as Miscdevice>::Ptr.

Thanks!

>
> > > +        kernel::build_error(VTABLE_DEFAULT_ERROR)
> >
> > What is this? Is this not yet implemented or something, and this is a
> > placeholder or something?
>
> It's a build-time assertion that this function is dead-code eliminated.
>
> There are two cases:
> * Either the driver does not override mmap. In this case, we store a
> null pointer in the fops table.
> * Or the driver overrides mmap with their own implementation. In this
> case, we store a function pointer to whichever implementation they
> provided.
> In neither case is the above function present anywhere in the final executable.

Ah nice, zero-cost stuff.

>
> Alice