[wahern]

Well, this OS in Rust project simply commits the same sins as the standard library by using a custom allocation routine that panics on allocation failure: https://os.phil-opp.com/heap-allocation/#allocations-in-rust

Saying that Rust the language doesn't require a non-failing allocator misses the point--the ergonomics of the language make dealing with allocation failure difficult; sufficiently difficult that none of the projects I've seen actually bother attempting it.

See https://cs.brown.edu/research/pubs/theses/ugrad/2015/light.a..., which explains idiomatic Rust instructs developers to return by value, relying on caller assignment to types like Box (which uses exchange_malloc under the hood), to handle heap allocation. Basically, the strategy for dynamic object management in Rust is predicated on hidden heap allocations.

So of course it's not necessary. But good luck writing an entire operating system otherwise. Even Redox OS doesn't bother trying to fight the language in this regard: https://gitlab.redox-os.org/redox-os/slab_allocator/blob/mas...

[comex]

That's not quite right. Rust has two ways to allocate a Box.

One is the `box` keyword: this guarantees that the object will be constructed in place on the heap, but doesn't work with fallible allocators (or, in fact, anything but the default allocator). This is what the research paper you linked is talking about. However, all these years later, `box` has never been stabilized; nor will it be in its current form, because it's considered too inflexible. Whatever form of in-place construction does eventually get stabilized will likely support fallibility.

The other way to allocate a Box is `Box::new`. This is not compiler magic; it's simply a regular function, implemented in Rust, that calls the allocator and then moves (i.e. memcpys) an existing object into the new allocation. If you write your own Box-like type, there's nothing stopping you from making your `new` function fallible.

What about optimizations? Does `Box::new` get optimized in ways that a fallible version won't? Well, no. The compiler will inline `Box::new`, and if you call it with a fresh stack allocation as an argument, LLVM can theoretically, sometimes, elide the stack allocation and the memcpy altogether, instead initializing the object directly on the heap. Theoretically. The paper claims that it always does so, but the paper is wrong. [1] In fact, the compiler doesn't do so even in relatively easy cases. [2] It would be nice if LLVM did better here, but it doesn't seem to be a big source of overhead in Rust programs in practice. If LLVM did improve the optimization, it would probably work equally well for a fallible allocator as for `Box::new`, because they're equally complex from its perspective: `Box::new` can panic, and LLVM treats panics as branches.

(Box does have compiler magic for a different case: the ability to move out of it. Not being able to replicate that in a custom type is suboptimal, but not the end of the world.)

As for why those OS projects panic on allocation failure:

The phil-opp.com one implements the standard allocator interface in order to use the standard library container types. I think it sucks that Rust's standard containers don't support allocation failure, but you don't need to use them...

For Redox... I'm not actually sure what they're doing, but I think "fn oom" is a hook for users of the allocator to call if they want to panic on out-of-memory, not something that mandates panicking. At least, that's the behavior of `std::alloc::handle_alloc_error` [3], which was moved there from being a trait method on `GlobalAlloc` named `oom`. However, they're implementing a different `oom` on an old version of the `Alloc` (not `GlobalAlloc`) trait, which is unstable; that method was removed entirely well over a year ago, so I guess the code must be out of date.

[1] https://users.rust-lang.org/t/how-to-create-large-objects-di...

[2] https://play.rust-lang.org/?version=stable&mode=release&edit... (press "..." -> "ASM")

[3] https://doc.rust-lang.org/nightly/std/alloc/fn.handle_alloc_...

[jimktrains2]

> the ergonomics of the language make dealing with allocation failure difficult; sufficiently difficult that none of the projects I've seen actually bother attempting it.

Honest question: what languages make this ergonomic and can you share any projects that handle this gracefully?

[wahern]

Off-hand I don't know of any that make this ergonomic, at least none that don't make use of exceptions. Rust isn't unique in this regard.

What makes it a potential impediment in Rust is that the constraints and burdens of the borrow checker are offset by mechanisms like Box. The fact that all the extant examples choose the convenience of Box over handling OOM, even in situations where not handling OOM is obviously a deal breaker for production systems, speaks volumes about the significance of the problem and how the language shapes people's choices. Async/await is in the same boat--technically doesn't require a non-failing heap allocation, but who's going to bother making it work? You technically don't need async/await to do asynchronous programming, either, but the whole point was that this is the type of thing that needs to be addressed by the core language with some primitive (i.e. generators) that does the heavy lifting and which can be built upon.

I don't know enough Rust to know how easy it would be to create a Box-like implementation that rewrites the AST to automatically propagate allocation failure via the idiomatic Result<T, E> protocol. But that seems roughly what the proper solution might look like in Rust; either that or finally getting over the anxiety and aversion about exceptions.

Lua handles OOM quite well. Lua doesn't have try/catch, just "protected calls" which are not as light-weight as regular function calls. (A pcall initializes a recovery point with _setjmp.) In Lua you tend to use protected calls at, effectively, transactional boundaries--the points in your call graph where you're willing and able to rollback application state for non-specific, otherwise unrecoverable errors. AFAIU you could technically do the same in Rust, except Rust makes unwinding optional at compile-time[1], so it's not the kind of thing people will make a habit of deliberately designing for in their libraries. Which makes me think the likely solution for Rust, if any, is to permit libraries to opt-in to OOM recovery with an allocator pragma that does something like the Result<T, E> propagation mentioned above.

[1] Lua is GC'd. The cost of running destructors outside the normal call/return protocol is fixed and independent of whether an application uses protected calls.

[monocasa]

Writing a KBox that calls a kmalloc and where new() returns a Result<KBox<T>, KAllocFailure> should be pretty trivial.