Re: [PATCH v6 4/4] rust: add abstraction for `struct page`

[Benno Lossin]

On 19.04.24 01:04, Boqun Feng wrote:
> On Thu, Apr 18, 2024 at 03:56:11PM -0700, Boqun Feng wrote:
>> On Thu, Apr 18, 2024 at 10:08:40PM +0000, Benno Lossin wrote:
>>> On 18.04.24 20:52, Boqun Feng wrote:
>>>> On Thu, Apr 18, 2024 at 08:59:20AM +0000, Alice Ryhl wrote:
>>>>> +    /// Runs a piece of code with a raw pointer to a slice of this page, with bounds checking.
>>>>> +    ///
>>>>> +    /// If `f` is called, then it will be called with a pointer that points at `off` bytes into the
>>>>> +    /// page, and the pointer will be valid for at least `len` bytes The pointer is only valid on
>>>>> +    /// this task, as this method uses a local mapping.
>>>>> +    ///
>>>>> +    /// If `off` and `len` refers to a region outside of this page, then this method returns
>>>>> +    /// `EINVAL` and does not call `f`.
>>>>> +    ///
>>>>> +    /// # Using the raw pointer
>>>>> +    ///
>>>>> +    /// It is up to the caller to use the provided raw pointer correctly. The pointer is valid for
>>>>> +    /// `len` bytes and for the duration in which the closure is called. The pointer might only be
>>>>> +    /// mapped on the current thread, and when that is the case, dereferencing it on other threads
>>>>> +    /// is UB. Other than that, the usual rules for dereferencing a raw pointer apply: don't cause
>>>>> +    /// data races, the memory may be uninitialized, and so on.
>>>>> +    ///
>>>>> +    /// If multiple threads map the same page at the same time, then they may reference with
>>>>> +    /// different addresses. However, even if the addresses are different, the underlying memory is
>>>>> +    /// still the same for these purposes (e.g., it's still a data race if they both write to the
>>>>> +    /// same underlying byte at the same time).
>>>>> +    fn with_pointer_into_page<T>(
>>>>> +        &self,
>>>>> +        off: usize,
>>>>> +        len: usize,
>>>>> +        f: impl FnOnce(*mut u8) -> Result<T>,
>>>>
>>>> I wonder whether the way to go here is making this function signature:
>>>>
>>>>       fn with_slice_in_page<T> (
>>>>           &self,
>>>> 	       off: usize,
>>>> 	       len: usize,
>>>> 	       f: iml FnOnce(&UnsafeCell<[u8]>) -> Result<T>
>>>>       ) -> Result<T>
>>>>
>>>> , because in this way, it makes a bit more clear that what memory that
>>>> `f` can access, in other words, the users are less likely to use the
>>>> pointer in a wrong way.
>>>>
>>>> But that depends on whether `&UnsafeCell<[u8]>` is the correct
>>>> abstraction and the ecosystem around it: for example, I feel like these
>>>> two functions:
>>>>
>>>> 	    fn len(slice: &UnsafeCell<[u8]>) -> usize
>>>> 	    fn as_ptr(slice: &UnsafeCell<[u8]>) -> *mut u8
>>>>
>>>> should be trivially safe, but I might be wrong. Again this is just for
>>>> future discussion.
>>>
>>> I think the "better" type would be `&[UnsafeCell<u8>]`. Since there you
>>> can always access the length.
>>>
>>
>> Hmm.. here is the thing, having `&UnsafeCell<[u8]>` means having a `*mut
>> [u8]>`, and it should always be safe to get a "length" of `*mut [u8]`,
>> right? I haven't found any method doing that, but the length should be
>> just a part of fat pointer, so I think getting that is a defined
>> behavior. But maybe I'm missing something.

There is `to_raw_parts` [1], but that is unstable. (Note that
`<[T] as Pointee>::Metadata = usize`, see [2])

>>
> 
> Hmm... but I guess one of the problems of this approach, is how to
> construct a `&UnsafeCell<[u8]>` from a pointer and length...

We could use `from_raw_parts` [3]. But when making the slice the outer
type, we can use a stable function to convert a pointer and a length to
a slice [4].

> 
> Regards,
> Boqun
> 
>>> Another question would be if page allows for uninitialized bits, in that
>>> case, we would need `&[Opaque<u8>]`.
>>>
>>
>> Yes, or `&Opaque<[u8>]`.

I don't think that putting the slice on the inside is what we want. Also
note that `Opaque<T>` requires that `T: Sized` and that is not the case
for `[u8]`.

[1]: https://doc.rust-lang.org/nightly/core/primitive.pointer.html#method.to_raw_parts
[2]: https://doc.rust-lang.org/nightly/core/ptr/trait.Pointee.html#pointer-metadata
[3]: https://doc.rust-lang.org/nightly/core/ptr/fn.from_raw_parts.html
[4]: https://doc.rust-lang.org/nightly/core/slice/fn.from_raw_parts.html

-- 
Cheers,
Benno