On Tue, 11 Nov 2025 14:11:28 GMT, Harald Eilertsen <[email protected]> wrote:
> `jdk.internal.foreign.SegmentFactories::allocateNativeInternal` assumes that > the underlying implementation of malloc aligns allocations on 16 byte > boundaries for 64 bit platforms, and 8 byte boundaries on 32 bit platforms. > So for any allocation where the requested alignment is less than or equal to > this default alignment it makes no adjustment. > > However, this assumption does not hold for all allocators. Specifically > jemallc, used by libc on FreeBSD will align small allocations on 8 or 4 byte > boundaries, respectively. This causes allocateNativeInternal to sometimes > return memory that is not properly aligned when the requested alignment is > exactly 16 bytes. > > To make sure we honour the requested alignment when it exaclty matches the > quantum as defined by MAX_MALLOC_ALIGN, this patch ensures that we adjust the > alignment also in this case. > > This should make no difference for platforms where malloc allready aligns on > the quantum, except for a few unnecessary trivial calculations. > > This work was sponsored by: The FreeBSD Foundation >From a logical point of view, what we'd need would be a couple of extra >constants: * `MIN_ALIGN`, this is the minimum alignment provided by the allocator/OS/platform combo * `MAX_ALIGN`, this is the maximum alignment provided by the allocator/OS/platform combo Then, we have three cases: * if the requested alignment `A` is `A <= MIN_ALIGN`, we can just allocate and don't adjust for alignment * if the requested alignment `A` is `MIN_ALIGN < A <= MAX_ALIGN` and the requested size is a multiple of the alignment, also just allocate and don't adjust for alignment * otherwise, allocate a bigger segment and manually align the result The problem is: how do we discover these constants? > Having something like os::posix_memalign() could eliminate the problem > completely, and probably simplify the code in allocateNativeInternal quite a > bit. Yeah, that would be nice -- but I noticed that `posix_memalign` is currently not allowed in hotspot code: https://github.com/openjdk/jdk/blob/400a83da893f5fc285a175b63a266de21e93683c/src/hotspot/os/posix/forbiddenFunctions_posix.hpp#L45 So, allowing this would require some discussion. Also, going down this path will likely require its own `Unsafe` primitive, and intrinsics, plus potential tweaks to support NMT. So, not straightforward to pull off. ------------- PR Comment: https://git.openjdk.org/jdk/pull/28235#issuecomment-3521432776
