Summary
-------

Obsolete the `UseSHM` and `UseHugeTLBFS` flags in JDK 22 and expire them in JDK 23.

Problem
-------

On Linux, with `-XX:+UseLargePages`, the JVM supports two modes to allocate static (non-transparent) hugepages: 

- using `mmap`(2), controlled by `UseHugeTLBFS`
- using `shmget`(2), controlled by `UseSHM`

Keeping both switches and code paths is an unnecessary technical debt and increases maintenance costs.

Both ways control the same backend functionality in the kernel [1]. The only difference is the API entry point. Short history - please see [2] for more details:

The `shmget`-based implementation came first, preceding the initial OpenJDK code drop. 

With JDK 6, JDK-7034464 [3][4] introduced `mmap`-based static hugepage allocation. The old `shmget` code path was preserved as a precaution. Two switches were introduced: `UseSHM` and `UseHugeTLBFS`. One is the negation of the other (in hindsight, a single boolean or mode switch would have been better).

Since JDK 6, `UseHugeTLBFS` had been the default, `UseSHM` the fallback if `mmap` failed. 

Both hugepage allocation paths are equally well supported since Linux 2.6. That is ancient and EOL.

I believe that in practice `UseSHM` - and hence `shmget` - usage dropped to almost zero since JDK 6 because:

- there is no benefit of explicitly enforcing `-XX:+UseSHM`
- situations where `UseHugeTLBFS` would fail but `UseSHM` succeed are unknown - in fact, it can be the other way around since `shmget` may lack permissions that `mmap` does not need.

Since `UseSHM` has been rarely used, it is prone to bitrot. In addition, the semantics of using large page reservations with `shmget` differs from `mmap`, since `shmget` reservations cannot be split. But that property is not well abstracted for upper layers, and there may be broken corner cases (e.g., in NMT).

Solution
--------

Remove the coding underlying `UseSHM`.

Obsolete `UseSHM`.

Since the only remaining implementation is the one backing `UseHugeTLBFS`, and there is no alternative, we can also obsolete `UseHugeTLBFS`.

Specification
--------

```
diff --git a/src/hotspot/os/linux/globals_linux.hpp b/src/hotspot/os/linux/globals_linux.hpp
index 9dc070233fe..aa419cd0d25 100644
--- a/src/hotspot/os/linux/globals_linux.hpp
+++ b/src/hotspot/os/linux/globals_linux.hpp
@@ -44,18 +44,12 @@
   product(bool, UseLinuxPosixThreadCPUClocks, true,                     \
           "enable fast Linux Posix clocks where available")             \
                                                                         \
-  product(bool, UseHugeTLBFS, false,                                    \
-          "Use MAP_HUGETLB for large pages")                            \
-                                                                        \
   product(bool, UseTransparentHugePages, false,                         \
           "Use MADV_HUGEPAGE for large pages")                          \
                                                                         \
   product(bool, LoadExecStackDllInVMThread, true,                       \
           "Load DLLs with executable-stack attribute in the VM Thread") \
                                                                         \
-  product(bool, UseSHM, false,                                          \
-          "Use SYSV shared memory for large pages")                     \
-                                                                        \
   product(bool, UseContainerSupport, true,                              \
           "Enable detection and runtime container configuration support") \
                                                                         \
diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
index 210635f9bd2..d23eeaa7842 100644
--- a/src/hotspot/share/runtime/arguments.cpp
+++ b/src/hotspot/share/runtime/arguments.cpp
@@ -523,6 +523,10 @@ static SpecialFlag const special_jvm_flags[] = {
   { "G1ConcRSHotCardLimit",         JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
   { "RefDiscoveryPolicy",           JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
   { "MetaspaceReclaimPolicy",       JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
+#ifdef LINUX
+  { "UseHugeTLBFS",                 JDK_Version::undefined(), JDK_Version::jdk(22), JDK_Version::jdk(23) },
+  { "UseSHM",                       JDK_Version::undefined(), JDK_Version::jdk(22), JDK_Version::jdk(23) },
+#endif
 
 #ifdef ASSERT
   { "DummyObsoleteTestFlag",        JDK_Version::undefined(), JDK_Version::jdk(18), JDK_Version::undefined() },
```


- [1] https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt
- [2] https://mail.openjdk.org/pipermail/hotspot-dev/2023-June/075924.html
- [3] https://bugs.openjdk.org/browse/JDK-7034464
- [4] https://github.com/openjdk/jdk6/commit/c796d80b281c81bee156919378d9afec8c85c220