The kernel radar: folios, multi-generational LRU, and Rust

The kernel radar: folios, multi-generational LRU, and Rust

As a side note, the Rust team are working on providing a decent API for fallible allocations. The raw Allocator trait that's going to be the interface between allocators and the rest of the language only has fallible functions that may allocate. The only function in that API that must succeed is free.

Then, there's effort ongoing to support fallible allocation in container APIs, with its own tracking issue.

There's a lot of effort going in to making this a supportable API for the long term; the key "odd" decision Rust seems to be making here is that instead of requiring every operation that could allocate to cope with the risk of allocation failure, there will be a mechanism (try_reserve on collections, for example) that guarantees that you can do a certain number of operations without allocating, and that mechanism can fail. It's then up to the users of the APIs to ensure that memory is either reserved up-front, or to take the risk of surprise allocation failure.

And this matches the underlying observation you've made; it's incredibly hard to handle allocation failure if it can happen at any time, and thus the entire application needs to either be bug-free in handling the rare error case (allocation failure), which in C++ means that everything has to meet the strong exception safety guarantee, or you need to confine allocation failures to a few places where you have hardened your code.