RFR: 8367485: os::physical_memory is broken in 32-bit JVMs when running on 64-bit OSes [v3]

[Joel Sikström]

On Thu, 18 Sep 2025 13:49:59 GMT, Anton Artemov <duke at openjdk.org> wrote:

>> Hi, please consider the following changes:
>> 
>> In this PR we address the overflow issue in `os::physical_memory()` on Linux, which can occur when running a 32-bit JVM on a 64-bit machine, introduced by https://bugs.openjdk.org/browse/JDK-8357086. The problem is that the product of _SC_PHYS_PAGES and _SC_PAGESIZE can overflow according to the documentation. 
>> 
>> The issue is addressed by changing the output type of all related functions to `uint64_t`.
>> 
>> Tested in tiers 1 - 5.
>
> Anton Artemov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8367485: Addressed reviewer's comments.

src/hotspot/share/runtime/os.cpp line 1944:

> 1942:   //     We allow some part (1/8?) of the memory to be "missing",
> 1943:   //     based on the sizes of DIMMs, and maybe graphics cards.
> 1944:   const uint64_t missing_memory   = 256UL * M;

There was some alignment going on here. Either we keep it or we might as well remove the spacing.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27335#discussion_r2359431693