[uecker]

No, but jemalloc uses a kernel API that has the behavior and IMHO is is then non-conforming (when using this API, which I think is configurable). The Facebook bug should be taken as a clear sign that this behavior is a terrible idea and not something to be even blessed by modifying the standard. When the original kernel API was introduced, it was already pointed out that the behavior is not ideal. There is no fundamental reason (including performance reasons) this has to behave in this way. It is just bad engineering.

[tialaramex]

But far from "The compiler shouldn't allow this" what we're talking about here is platform behaviour. My impression is that virtual machines often just do this, so it may be that even your OS has no idea either.

[uecker]

Virtual machines do not change memory behind your back without your permission.. The issue with jemalloc is very specific problem with a specific Linux API, i.e. MADV_FREE that has the problematic behavior, i.e. it reallocates pages when written-to but not already when accessed. When using this API, jemalloc is not conforming implementation of malloc. We can not weaken semantics of language semantics everytime someone implements something broken. Why MADV_FREE behaves like this is unclear to me, it was criticized the moment it was introduced into the kernel. But the main problem is using it for a memory allocator in C.