RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v4]

[Jorn Vernee]

On Sun, 16 Nov 2025 18:09:47 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:
> 
>   Fix calculation of allocationSize when byteAlignment < MAX_MALLOC_ALIGN
>   
>   This work was sponsored by: The FreeBSD Foundation

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 - Math.min(MAX_MALLOC_ALIGN, alignedSize);

This doesn't look correct to me. Let's say we want to do a 7 byte allocation (with `init == false`), aligned to 4. We get `7 % 4 -> 3` so we enter this branch, and then `allocationSize = 7 + 4 - 7`, and we end up allocating only 4 bytes. The previous computation only works if `MAX_MALLOC_ALIGN` is greater than `byteAlignment`.

I think it's simpler to just replace `MAX_MALLOC_ALIGN` in this `if` statement with an 'expected alignment' derived from the size.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28235#discussion_r2534441590