RFR: JDK-8281023: NMT integration into pp debug command does not work

[Zhengyu Gu]

On Tue, 1 Feb 2022 08:36:42 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> JDK-8280289 enhanced the debug pp() command to use NMT if enabled, and to print NMT related info. That is useful, but there are some issues.
> 
> On debug, it just asserts, since the empty reserved region we create to hold the output of the mmap-search is created with address=NULL:
> 
> 
> (gdb) call pp(0x7ffff010b030)
> 
> "Executing pp"
> 
> Thread 2 "java" received signal SIGSEGV, Segmentation fault.
> 0x00007ffff6721a71 in VirtualMemoryRegion::VirtualMemoryRegion (this=this at entry=0x7ffff5bb2620, addr=addr at entry=0x0, size=size at entry=0) at /shared/projects/openjdk/jdk-jdk/source/src/hotspot/share/services/virtualMemoryTracker.hpp:180
> 180 assert(addr != NULL, "Invalid address");
> 
> 
> On release we don't assert and get further, but the use of SafeFetch is slightly wrong. It will deny us any NMT data about p if *p==0:
> 
> 
> if (CanUseSafeFetchN() && SafeFetchN((intptr_t*)p, 0) != 0) {
> 
> 
> This patch:
> - fixes uses of SafeFetch
> - changes the mmap-region-search-code to not require an empty ReservedMemoryRegion in order to avoid triggering the assert in virtualMemoryTracker.hpp:180
> - adds a comment about the safe use of pp() in gdb (one needs to switch off signal handling of SIGSEGV for this to work)
> 
> Tests:
> - I tested manually that pp works with different levels of NMT (Linux x64)
> - GHAs in process

Changes requested by zgu (Reviewer).

src/hotspot/share/services/virtualMemoryTracker.cpp line 699:

> 697:   walk_virtual_memory(&walker);
> 698:   return walker.region();
> 699: }

Snapshot the region is for avoiding race pointed out by Ioi in code review, because other thread might release the region after walk.

src/hotspot/share/utilities/debug.cpp line 505:

> 503:       //  is handled quietly by the VM, but it will trip up the debugger. gdb will catch the signal and disable
> 504:       //  the pp() command for further use.
> 505:       // In order to avoid that, before invoking pp(), switch off SIGSEGV handling with "handle SIGSEGV nostop".

Ah, my .gdbinit has `handle SIGSEGV nostop noprint pass`

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PR: https://git.openjdk.java.net/jdk/pull/7297