RFR: 8353564: Fail fatally if os::release_memory or os::uncommit_memory fails [v7]

[Robert Toyonaga]

On Thu, 29 Jan 2026 18:45:05 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

>> This PR is a follow up to JDK-8341491. See original discussion: https://github.com/openjdk/jdk/pull/24084#issuecomment-2752513700
>> 
>> This PR makes `os::release_memory`, `os::uncommit_memory`, `os::release_memory_special`, and `os::unmap_memory` fail fatally if they encounter an error. These methods require obtaining the NMT lock. Fatally failing in these places would potentially allow for the tightening of NMT virtual memory locking scopes (future work, if this PR is accepted).  Already in most cases, the callers fail fatally or assert(false) when these os:: methods fail. Another reason to fatally fail is that if the OS memory operation fails, it can be difficult to know for sure what state the OS left the memory in and recover. 
>> 
>> `release_memory`/`uncommit_memory`/`release_memory_special`/`unmap_memory` can fail due to ① Bad arguments, or ② The OS encountered an issue out of control of the JVM.
>> 
>> ①
>> If there is a JVM bug, it's probably reasonable to fatally fail here. Or the caller could be intentionally passing arguments that may or may not be valid. I don't think there is any code like that currently, and this is probably a bad pattern to be following anyway.
>> 
>> ②
>> In platform dependent code: 
>> With regard to mmap/munmap, the only errors that aren't due to bad arguments are ENOMEM and ones related to file descriptors (which are not applicable to uncommit or release).
>> On Windows, VirtualFree only fails due to bad arguments.
>> On AIX, shmdt and disclaim64 only fail due to bad arguments. msync could spontaneously fail with EIO: "An I/O error occurred while reading from or writing to the file system."
>> On BSD, it seems like mprotect and madvise fail only due to bad arguments or invalid privileges.
>> 
>> In the [original discussion](https://github.com/openjdk/jdk/pull/24084#issuecomment-2752513700), the main question was whether ENOMEM upon os::uncommit_memory was recoverable. This may be possible if we uncommit the middle of a region - splitting it in two. This could exceed the limit of the number of mappings resulting in ENOMEM. 
>> 
>> If none of the scenarios in ② are recoverable, then perhaps fatally failing is OK. 
>> 
>> Testing:
>> 	- Tier 1.
>> 	- Manual testing to make sure we fatally fail and the correct messages are printed.
>
> Robert Toyonaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   stray whitespace

I think that all the review feedback has been resolved. Now the main question remaining is whether it's good to fail immediately in the os:: memory functions.

Reasons for: 
- Doing this can help tighten NMT locking scope. 
- It's better to fail immediately upon encountering a problem rather than scrape along and fail somewhere else. It could make diagnosing problems easier.

Reasons against:
- Maybe callers should have control over how they deal with failures
  - However, the memory may be in an unknown/unreliable state now (not recoverable). So execution of any handling in the caller is unreliable. 
- If the OS calls change in the future and become capable of failing for new reasons that happen to be recoverable, we'll have to undo this work. However, I'm not sure if this is a very likely scenario.

@tstuefe  and @stefank, Can you please have another look when you have time?

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PR Comment: https://git.openjdk.org/jdk/pull/29240#issuecomment-3878353215