RFR: 8292083: Detected container memory limit may exceed physical machine memory [v3]

[Severin Gehwolf]

On Wed, 17 Aug 2022 13:54:09 GMT, Jonathan Dowland <jdowland at openjdk.org> wrote:

>> We discovered some systems configured with cgroups v1 which report a bogus container memory limit value which is above the physical memory of the host. OpenJDK then calculates flags such as InitialHeapSize based on this invalid value; this can be larger than the available memory which can result in the OS terminating the process due to OOM.
>> 
>> hotspot's container awareness attempts to sanity check the limit value by ensuring it's below `_unlimited_memory = (LONG_MAX / os::vm_page_size()) * os::vm_page_size()`, but that still leaves a large range of potential invalid values between physical RAM and that ceiling value.
>> 
>> Cgroups V1 in particular returns an uninitialised value for the memory limit when one has not been explicitly set. Cgroups v2 does not suffer the same problem: however, it's possible for any value to be set for the max memory, including values exceeding the available physical memory, in either v1 or v2.
>> 
>> This fixes the problem in two places. Further work may be required in the area of Java metrics / MXBeans. I'd also look again at whether the existing ceiling value `_unlimited_memory` serves any useful purpose. I personally don't feel those improvements should hold up this fix.
>
> Jonathan Dowland has updated the pull request incrementally with one additional commit since the last revision:
> 
>   restructure conditions for legibility
>   
>    * split assignment to mem_limit from reading it
>    * nest if expressions to avoid comparing mem_limit twice

Come to think of this some more the better way to fix this would be to change `OSContainer::memory_limit_in_bytes()` and return `-1` (for unlimited) if the detected limit exceeds host memory.

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PR: https://git.openjdk.org/jdk/pull/9880