RFR: 8261023: Document why memory pretouch must be a store [v2]

[Thomas Schatzl]

> Hi all,
> 
>   may I have reviews for this additional comment that explains why `os::pretouch_memory` needs to use a store and must not use a read which would be more convenient?
> 
> Basically on some (all?) OSes memory pages are only actually backed with physical memory on a store to that page. Before that a common "zero page" may be used to satisfy reads. This is not what is intended here.
> 
> A previous comment (that has been removed long ago) seems to have been a bit confused about the actual issue:
> 
> -      // Note the use of a write here; originally we tried just a read, but
> -      // since the value read was unused, the optimizer removed the read.
> -      // If we ever have a concurrent touchahead thread, we'll want to use
> -      // a read, to avoid the potential of overwriting data (if a mutator
> -      // thread beats the touchahead thread to a page).  There are various
> -      // ways of making sure this read is not optimized away: for example,
> -      // generating the code for a read procedure at runtime.
> 
> It indicates that the reason for using a store has been that the compiler would optimize away the reads (which begs the question why a `volatile` read has not been used).
> 
> Maybe these zero page optimizations came later than that original implementation though.
> 
> Testing: local compilation - it's adding a comment only, really.
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  shade review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2373/files
  - new: https://git.openjdk.java.net/jdk/pull/2373/files/d30fff80..2009527e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2373&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2373&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2373.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2373/head:pull/2373

PR: https://git.openjdk.java.net/jdk/pull/2373