RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v3]

[Jorn Vernee]

On Fri, 14 Nov 2025 13:13:00 GMT, Harald Eilertsen <haraldei at openjdk.org> 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
>
> Harald Eilertsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   OS agnostic fix for alignment of native segments
>   
>   Only align up the requested memory if the requested alignment is larget
>   than max alignment provided by malloc, or if the requested size is not a
>   multiple of the alignment size.
>   
>   This work was sponsored by: The FreeBSD Foundation
>   
>   Co-authored-by: mcimadamore

src/java.base/share/classes/jdk/internal/foreign/SegmentFactories.java line 207:

> 205:         long result;
> 206:         if (byteAlignment > MAX_MALLOC_ALIGN || alignedSize % byteAlignment != 0) {
> 207:             allocationSize = alignedSize + byteAlignment - MAX_MALLOC_ALIGN;

The calculation of `allocationSize` looks no longer correct now that `byteAlignment` might be smaller than `MAX_MALLOC_ALIGN`, and it can result in a negative size.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28235#discussion_r2530070997