RFR: 8275063: Implementation of Foreign Function & Memory API (Second incubator) [v12]

[Maurizio Cimadamore]

On Tue, 2 Nov 2021 18:55:47 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> I'll have to think some more about this. I don't think this is covered inside the block - that is, the block tries to allocate, and then in the finally we throw if we realized we've allocated too much.
>
> What is missing, I think, is a check (size > arenaSize) at the beginning of the method (we only check this in one of the paths). But we need to check before and after, I think, as it is possible to allocate a segment and then realize that we ended up overflowing the arena size.

While what I said above correctly reflects what the implementation does, I think a broader issue is that the arena allocator implementation is allocating sometimes more native memory than what its contract specifies. While in some cases we can prevent that, I think in the general case (e.g. where we allocate a new block) we cannot, unless we add extra API guarantees - e.g. that the arena size should be a multiple of the block size (but then we'd have to special case `Long.MAX_VALUE`, or maybe pick a "big enough" power of two instead)

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PR: https://git.openjdk.java.net/jdk/pull/5907