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abendstolz | 4 years ago

Oh, wow, I was under the impression that the error message would be stack only, no heap involved, but as Result is part of the std library and not of core, this totally makes sense.

So for `Rust for Linux` they also need to implement a `Result-like` type that is stack only based to solve this issue, right?

If so, cool, thanks, you just made my day by tickling my learning nerves! :)

discuss

order

cormacrelf|4 years ago

It has nothing to do with Result, whatsoever. Result does not allocate. If you used a Result that way, you could certainly try to "gracefully" handle the allocation failure, but if you think it would be easy, you would be wrong. As Tialaramex said, you are probably just going to make the problem worse because it is very difficult to ensure you do not attempt to allocate during allocation-failure-recovery. Rustc doesn't and can't really check this for you.

It actually has to do with `panic!(...)`. When you use `unwrap()`/`expect("...")`, you use the panic macro under the hood; parts of the panicking infrastructure use a boxed trait object which could contain a static string or formatted String or anything else really. The box can allocate if it is not a ZST. I believe the alloc crate's default handler tries to avoid this kind of thing, so that it can't fail to allocate AGAIN in the failure-handling routine. It will likely do a better job than you could.

This is a live issue at the moment, so to go into any more detail I'd have to read a bunch of recent Rust issues/PRs.

cormacrelf|4 years ago

An addendum to tie this back to the original discussion: the reason kernel devs want these APIs more than userland is that (a) in a kernel, panicking = crashing the computer, which would be bad, and (b) they have a much bigger toolbox for handling OOM.

They can kill entire misbehaving processes. What are you going to do in your little program, clear a cache whose objects are sprinkled evenly across 150 different pages? You would need more control than you get from blindly using malloc/free/rust_alloc globally. Something like memcached would be able to use these APIs, because it uses its own allocator, and knows enough about its layout to predictably free entire pages at once.

abendstolz|4 years ago

Thanks for the thorough explanation!