RFR: 8305770: os::Linux::available_memory() should refer MemAvailable in /proc/meminfo [v2]

[Yasumasa Suenaga]

> `os::Linux::available_memory()` returns available memory from cgroups or sysinfo(2). In case of the process which run on out of container, that value is based on `freeram` from sysinfo(2).
> 
> `freeram` is equivalent to `MemFree` in `/proc/meminfo` [1]. However it means just a free RAM. We should use `MemAvailable` when we want to know how much memory is available for the process [2]. `MemAvailable` is available in modern Linux kernel, and it has been backported some older kernels (e.g. RHEL). In `sar` from sysstat, it refers that value and shows it as `kbavail` [3].
> 
> AFAIK PhysicalMemory event in JFR depends on `os::Linux::available_memory()`, and it is used in automated analysis in JMC. So the JFR/JMC user could misunderstand physical memory was exhausted even if the memory was available enough.
> 
> [1] https://github.com/torvalds/linux/blob/c9c3395d5e3dcc6daee66c6908354d47bf98cb0c/fs/proc/meminfo.c#L59
> [2] https://docs.kernel.org/filesystems/proc.html?highlight=memavailable
> [3] https://github.com/sysstat/sysstat/blob/ac1df71ca252c158e8d418ded93e5ed52f5e8765/rd_stats.c#L325-L328

Yasumasa Suenaga has updated the pull request incrementally with one additional commit since the last revision:

  Introduce os::free_memory

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13398/files
  - new: https://git.openjdk.org/jdk/pull/13398/files/6e10eb9c..5bcac06b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13398&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13398&range=00-01

  Stats: 58 lines in 10 files changed: 45 ins; 3 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/13398.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13398/head:pull/13398

PR: https://git.openjdk.org/jdk/pull/13398