JvmtiExport::can_walk_any_space() usage in hotspot

[kalinshi(施慧)]

Hi hotspot experts,

Would you help on my question about JvmtiExport::can_walk_any_space() check?
Question is why JvmtiExport::can_walk_any_space() check is needed in CDS when mapping region?

JvmtiExport::can_walk_any_space() method is only used in FileMapInfo::map_region for modifing region read-only mapping attribute. 
JvmtiExport::can_walk_any_space() is set true when jvmtiCapabilities.can_tag_objects is enabled.
JVMTI capability can_tag_objects enables java heap iteration/object reference tracing, and JvmtiEnv::Set/GetTag doesn't modify read-only regions in shared archive (I might wrong).

comments in latest code seems outdated, JvmtiExport::can_walk_any_space() doesn't disable sharing now.
"
  JvmtiExport::set_can_walk_any_space(
    avail.can_tag_objects);   // disable sharing in onload phase
"

Back to initial code, class sharing is disabled when condition JvmtiExport::can_modify_any_class() || JvmtiExport::can_walk_any_space() is true.
This matches above comment in JvmtiManageCapabilities::update.
"
  if (JvmtiExport::can_modify_any_class() || JvmtiExport::can_walk_any_space()) {
    fail_continue("Tool agent requires sharing to be disabled.");
    return false;
  }
"

JvmtiExport::can_modify_any_class condition disables class data sharing when class file load hook (requires modify code and read only contents) is needed in initial code.
Both checks are removed and used to determine region read/write attribute with following commits. These commits are mainly supporting class file load hook with CDS.

1. enable shared class when these tow checks on, modify/map all regions in shared archive as RW.
   8054386: Allow Java debugging when CDS is enabled Map archive RW when debugging is enabled
   8087153: EXCEPTION_ACCESS_VIOLATION when CDS RO section vanished on win32
   
2. Support class file load hook with CDS
   8141341: CDS should be disabled if JvmtiExport::should_post_class_file_load_hook() is true Disable loading shared class if JvmtiExport::should_post_class_file_load_hook is true.
   8078644: CDS needs to support JVMTI CFLH Support posting CLFH for shared classes.
   
3. Fix jvmtiCapabilities::can_generate_all_class_hook_events inconsistent state when shared
   8161605: The '!UseSharedSpaces' check is not need in JvmtiManageCapabilities::recompute_always_capabilities
   
4. Fix class file load hook error for early class hook event when shared
   8212200: assert when shared java.lang.Object is redefined by JVMTI agent

Regards
Hui