RFR: 8234930: Use MAP_JIT when allocating pages for code cache on macOS

[David Holmes]

Hi Anton,

cc'ing hotspot-gc-dev as a number of GC files are touched, even though 
in a trivial way. It also broadens the scope for people who understand 
the detailed nuances of these mapping flags.

Cheers,
David

On 28/08/2020 11:57 pm, Anton Kozlov wrote:
> Hi,
> 
> Could you please review a change for
> 
> JDK-8234930: Use MAP_JIT when allocating pages for code cache on macOS
> Bug: https://bugs.openjdk.java.net/browse/JDK-8234930
> Webrev: http://cr.openjdk.java.net/~akozlov/8234930/webrev.00/
> 
> On macOS, MAP_JIT cannot be used with MAP_FIXED[1]. So pd_reserve_memory have
> to provide MAP_JIT for mmap(NULL, PROT_NONE), the function was made aware of
> exec permissions.
> 
> For executable and data regions, pd_commit_memory only unlocks the memory with
> mprotect, this should make no difference compared with old code.
> 
> For data regions, pd_uncommit_memory still uses a new overlapping anonymous
> mmap which returns pages to the OS and immediately reflects this in diagnostic
> tools like ps.  For executable regions it would require MAP_FIXED|MAP_JIT, so
> instead madvise(MADV_FREE)+mprotect(PROT_NONE) are used. They should also allow
> OS to reclaim pages, but apparently this does not happen immediately. In
> practice, it should not be a problem for executable regions, as codecache does
> not shrink (if I haven't missed anything, by the implementation and in
> principle).
> 
> Tested:
> * local tier1
> * jdk-submit
> * codesign[2] with hardened runtime and allow-jit but without
> allow-unsigned-executable-memory entitlements[3] produce a working bundle.
> 
> Thanks,
> Anton
> 
> [1] https://github.com/apple/darwin-xnu/blob/master/bsd/kern/kern_mman.c#L227
> [2]
>    codesign \
>      --sign - \
>      --options runtime \
>      --entitlements ents.plist \
>      --timestamp \
>      $J/bin/* $J/lib/server/*.dylib $J/lib/*.dylib
> [3]
>    <?xml version="1.0" encoding="UTF-8"?>
>    <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
>    <plist version="1.0">
>      <dict>
>        <key>com.apple.security.cs.allow-jit</key>
>        <true/>
>        <key>com.apple.security.cs.disable-library-validation</key>
>        <true/>
>        <key>com.apple.security.cs.allow-dyld-environment-variables</key>
>        <true/>
>      </dict>
>    </plist>
> 
> 
> 
>