RFR: 8292984: Refactor internal container-related interfaces for clarity

[Severin Gehwolf]

On Wed, 24 Sep 2025 13:55:35 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

> Hi everyone,
> 
> The current memory-related code paths in Linux are unclear and convoluted, with responsibilities and data flow crossing between `os::Linux` and various container-related layers.
> 
> For example, consider the call sequence for `os::available_memory()`:
> 
> os::available_memory()
>         |
>         v
> os::Linux::available_memory()
>         |--------------------------------------------
>         v                                           v    
> OSContainer::memory_limit_in_bytes()       or return host physical memory
>         |
>         v
> CgroupSubsystem::memory_limit_in_bytes()
>         |--------------------------------------------
>         v                                           v
> return os::Linux::physical_memory()        or return cgroup v1/v2 limit
> 
> 
> This structure is difficult to follow. Calls move between `os::Linux` and container subsystems in a confusing manner. Ideally, each component should be responsible only for its relevant functionality:
> * `os::Linux` should focus solely on actual machine memory values.
> * `CgroupSubsystem` should focus exclusively on cgroup memory limits.
> * The selection of which value to use should occur at the `os` layer, based on whether the environment is containerized.
> 
> 
> A revised structure separates these responsibilities:
> 
> os::available_memory()
>         |--------------------------------------------
>         v                                           v
> OSContainer::memory_limit_in_bytes()       or os::Linux::available_memory()                       
>         |--------------------------------------------
>         v                                           v
> CgroupSubsystem::memory_limit_in_bytes()   os::Linux::physical_memory()
>         |
>         v
> return bounded cgroup v1/v2 limit
> 
> 
> With these changes:
> * `os::Linux` only retrieves machine values.
> * `CgroupSubsystem` works exclusively with cgroup limits.
> * `OSContainer` fetches and passes bounds for the cgroup values.
> * The decision of container or machine value is done in the `os` layer.
> 
> The concrete code changes include:
> * Moving container selection logic from `os::Linux::{available/free}_memory()` to `os:{available/free}_memory()`, so `os::Linux` now only deals with machine values (was already the case for `os::physical_memory()`).
> * Moving `os::Linux::available_memory_in_container()` to `OSContainer` instead, removing container-specific logic from `os::Linux`. Also refactored to use the new bool and reference interface introduced in [JDK-8357086](https://bugs.openjdk...

Looks good to me, but should probably wait until [JDK-8367485](https://github.com/openjdk/jdk/pull/27335) is integrated.

-------------

Marked as reviewed by sgehwolf (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27470#pullrequestreview-3267322731