RFR: 8341491: Reserve and commit memory operations should be protected by NMT lock [v2]

[Robert Toyonaga]

On Wed, 2 Apr 2025 14:02:25 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 i...
>
> Robert Toyonaga has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - tests and comments
>  - Revert "make memory op and NMT accounting atomic"
>    
>    This reverts commit 86423d0b7e8e2b0b313a686a64c803028a5f2420.

OK I have reverted the original changes, added comments, and kept the new tests that are still relevant.  Please have another look when you have time.  I'll go ahead and open RFE's for the topics you suggested above. 
Thanks!

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PR Comment: https://git.openjdk.org/jdk/pull/24084#issuecomment-2772669368