Re: JvmtiExport::can_walk_any_space() usage in hotspot

[daniel . daugherty]

Adding serviceability-dev@... to this email thread since JVM/TI
is maintained by the Serviceability Team...

Dan


On 2/22/21 3:29 AM, kalinshi(施慧) wrote:

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comments in latest code seems outdated, JvmtiExport::can_walk_any_space() 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