RFR: 8292984: Refactor internal container-related interfaces for clarity [v2]

[Casper Norrbin]

On Tue, 30 Sep 2025 08:45:38 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 ref...
>
> Casper Norrbin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Merge branch 'master' into linux-container-mem-restructure
>  - keep cgroupsubsystem separate
>  - keep os::linux separate

Thank you for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/27470#issuecomment-3361266047