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    SubjectRe: [PATCH 1/7] rust: file: add Rust abstraction for `struct file`
    From
    Matthew Wilcox <willy@infradead.org>
    > I haven't looked at how Rust-for-Linux handles errors yet, but it's
    > disappointing to see that it doesn't do something like the PTR_ERR /
    > ERR_PTR / IS_ERR C thing under the hood.

    It would be cool to do that, but we haven't written the infrastructure
    to do that yet. (Note that in this particular case, the C function also
    returns the error as a null pointer.)

    >> @@ -157,6 +158,12 @@ void rust_helper_init_work_with_key(struct work_struct *work, work_func_t func,
    >> }
    >> EXPORT_SYMBOL_GPL(rust_helper_init_work_with_key);
    >>
    >> +struct file *rust_helper_get_file(struct file *f)
    >> +{
    >> + return get_file(f);
    >> +}
    >> +EXPORT_SYMBOL_GPL(rust_helper_get_file);
    >
    > This is ridiculous. A function call instead of doing the
    > atomic_long_inc() in Rust?

    I think there are two factors to consider here:

    First, doing the atomic increment from Rust currently runs into the
    memory model split between the C++ and LKMM memory models. It would be
    like using the C11 atomic_fetch_add instead of the one that the Kernel
    defines for LKMM using inline assembly. When I discussed this with Paul
    McKenney, we were advised that its best to avoid mixing the memory
    models.

    Avoiding this would require that we replicate the inline assembly that C
    uses to define its atomic operations on the Rust side. This is something
    that I think should be done, but it hasn't been done yet.


    Second, there's potentially an increased maintenance burden when C
    methods are reimplemented in Rust. Any change to the implementation on
    the C side would have to be reflected on the Rust side.

    Alice

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