Re: [PATCH 3/3] rust: add abstraction for `struct page`

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Feb 01, 2024 at 01:50:53AM -0500, Trevor Gross wrote:
> On Fri, Jan 26, 2024 at 1:28 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > On Fri, Jan 26, 2024 at 01:33:46PM +0100, Alice Ryhl wrote:
> > > On Fri, Jan 26, 2024 at 1:47 AM Boqun Feng <boqun.feng@gmail.com> wrote:
> > > >
> > > > On Wed, Jan 24, 2024 at 11:20:23AM +0000, Alice Ryhl wrote:
> > > > > [...]
> > > > > +    /// Maps the page and writes into it from the given buffer.
> > > > > +    ///
> > > > > +    /// # Safety
> > > > > +    ///
> > > > > +    /// Callers must ensure that `src` is valid for reading `len` bytes.
> > > > > +    pub unsafe fn write(&self, src: *const u8, offset: usize, len: usize) -> Result {
> > > >
> > > > Use a slice like type as `src` maybe? Then the function can be safe:
> > > >
> > > >         pub fn write<S: AsRef<[u8]>>(&self, src: S, offset: usize) -> Result
> > > >
> > > > Besides, since `Page` impl `Sync`, shouldn't this `write` and the
> > > > `fill_zero` be a `&mut self` function? Or make them both `unsafe`
> > > > because of potential race and add some safety requirement?
> > >
> > > Ideally, we don't want data races with these methods to be UB. They
> >
> > I understand that, but in the current code, you can write:
> >
> >         CPU 0                   CPU 1
> >         =====                   =====
> >
> >         page.write(src1, 0, 8);
> >                                 page.write(src2, 0, 8);
> >
> > and it's a data race at kernel end. So my question is more how we can
> > prevent the UB ;-)
> 
> Hm. Would the following work?
> 
>     // Change existing functions to work with references, meaning they need an
>     // exclusive &mut self
>     pub fn with_page_mapped<T>(
>         &mut self,
>         f: impl FnOnce(&mut [u8; PAGE_SIZE]) -> T
>     ) -> T
> 
>     pub fn with_pointer_into_page<T>(
>         &mut self,
>         off: usize,
>         len: usize,
>         f: impl FnOnce(&mut [u8]) -> Result<T>,
>     ) -> Result<T>
> 
>     // writing methods now take &mut self
>     pub fn write(&mut self ...)
>     pub fn fill_zero(&mut self ...)
>     pub fn copy_into_page(&mut self ...)
> 
>     // Add two new functions that take &self, but return shared access
>     pub fn with_page_mapped_raw<T>(
>         &self,
>         f: impl FnOnce(&UnsafeCell<[u8; PAGE_SIZE]>) -> T
>     ) -> T
> 
>     pub fn with_pointer_into_page_raw<T>(
>         &self,
>         off: usize,
>         len: usize,
>         f: impl FnOnce(&[UnsafeCell<u8>]) -> Result<T>,
>     ) -> Result<T>
> 
> This would mean that anyone who can obey rust's mutability rules can
> use a page without any safety or race conditions to worry about, much
> better for usability.
> 
> But if you do need to allow the data to be shared and racy, such as
> the userspace example, the `_raw` methods allow for that and you can
> `.get()` a `*mut u8` from the UnsafeCell. This moves the interior
> mutability only to the mapped data rather than the Page itself, which
> I think is more accurate anyway.
> 

Looks good to me ;-) Thanks!

Regards,
Boqun

> Leveraging UnsafeCell would also make some things with UserSlicePtr
> more clear too.
> 
> - Trevor
> 
> > Regards,
> > Boqun
> >
> > > could be mapped into the address space of a userspace process.
> > >
> > > Alice
> >
>