RFR: 8341491: Reserve and commit memory operations should be protected by NMT lock

[Robert Toyonaga]

On Mon, 17 Mar 2025 16:20:42 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

> ### Summary:
> This PR makes memory operations atomic with NMT accounting.
> 
> ### The problem:
> In memory related functions like `os::commit_memory` and `os::reserve_memory` the OS memory operations are currently done before acquiring the the NMT mutex. And the the virtual memory accounting is done later in `MemTracker`, after the lock has been acquired. Doing the memory operations outside of the lock scope can lead to races. 
> 
> 1.1 Thread_1 releases range_A.
> 1.2 Thread_1 tells NMT "range_A has been released".
> 
> 2.1 Thread_2 reserves (the now free) range_A.
> 2.2 Thread_2 tells NMT "range_A is reserved".
> 
> Since the sequence (1.1)  (1.2) is not atomic, if Thread_2 begins operating after (1.1), we can have (1.1) (2.1) (2.2) (1.2). The OS sees two valid subsequent calls (release range_A, followed by map range_A). But NMT sees "reserve range_A", "release range_A" and is now out of sync with the OS.
> 
> ### Solution:
> Where memory operations such as reserve, commit, or release virtual memory happen,  I've expanded the scope of `NmtVirtualMemoryLocker` to protect both the NMT accounting and the memory operation itself. 
> 
> ### Other notes:
> I also simplified this  pattern found in many places:
> 
> if (MemTracker::enabled()) {
>   MemTracker::NmtVirtualMemoryLocker nvml;
>   result = pd_some_operation(addr, bytes);
>   if (result != nullptr) {
>     MemTracker::record_some_operation(addr, bytes);
>   }
> } else {
>   result = pd_unmap_memory(addr, bytes);
> }
> ``` 
> To:
> 
> MemTracker::NmtVirtualMemoryLocker nvml;
> result = pd_unmap_memory(addr, bytes);
> MemTracker::record_some_operation(addr, bytes);
> ``` 
> This is possible because `NmtVirtualMemoryLocker` now checks `MemTracker::enabled()`.  `MemTracker::record_some_operation` already checks `MemTracker::enabled()` and checks against nullptr.  This refactoring previously wasn't possible because `ThreadCritical` was used before https://github.com/openjdk/jdk/pull/22745 introduced `NmtVirtualMemoryLocker`. 
> 
> I considered moving the locking and NMT accounting down into platform specific code:  Ex. lock around { munmap() + MemTracker::record }. The hope was that this would help reduce the size of the critical section. However, I found that the OS-specific "pd_" functions are already short and to-the-point, so doing this wasn't reducing the lock scope very much. Instead it just makes the code more messy by having to maintain the locking and NMT accounting in each platform specific implementation. 
> 
> In many places I've done minor refactoring by relocating call...

OK should I update this PR to do the following things:
-  Add comments explaining the asymmetrical locking and warning against patterns that lead to races
- swapping the order of `NmtVirtualMemoryLocker` and release/uncommit
- Fail fatally if release/uncommit does not complete.

Or does it make more sense to do that in a different issue/PR?

Also, do we want to keep the new tests and the refactorings (see below)?

if (MemTracker::enabled()) {
  MemTracker::NmtVirtualMemoryLocker nvml;
  result = pd_some_operation(addr, bytes);
  if (result != nullptr) {
    MemTracker::record_some_operation(addr, bytes);
  }
} else {
  result = pd_unmap_memory(addr, bytes);
}

To:

MemTracker::NmtVirtualMemoryLocker nvml;
result = pd_unmap_memory(addr, bytes);
MemTracker::record_some_operation(addr, bytes);

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

PR Comment: https://git.openjdk.org/jdk/pull/24084#issuecomment-2766365411