RFR: 8319875: Add macOS implementation for jcmd System.map [v2]

[Thomas Stuefe]

On Thu, 5 Dec 2024 14:32:53 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Hi @stooke !
>> 
>>> Hello, @tstuefe , and thanks for your comments. I'll address a few here while I work on the others. I have changed the os-specific names to lowercase, but I don't think it makes them stand out more. The square brackets were intended to do that. Might I change this back?
>> 
>> Sure, if it looks worse. I just wanted to make sure we can cleanly distinguish NMT sections from OS sections.
>> 
>>> 
>>> I think there is only one JAVAHEAP segment because due to an issue with my build[1] there was no CDS archive available.
>> 
>> Has nothing to do with CDS. The heap consists of committed and reserved areas. Committed areas have backing swap space allocated for them, and are accessible. Reserved areas have not and are generally not. API wise the difference is that Reserved sections set the MAP_NORESERVE flag for mmap, and are generally allocated with PROT_NONE. 
>> 
>> So, the heap should show up with several neighboring sections, some committed, some just reserved. Similar how most of the stacks should show up with two entries, one for the writable stack, one for the guard page that is protected.
>> 
>> ---
>> 
>> 
>> Simple test I did on MacOS with your patch: I reserve 1G of memory at startup, uncommitted (added to os::init_2)
>> 
>> 
>>   if (UseNewCode) {
>>     char* p = os::reserve_memory(G, false, mtInternal);
>>     tty->print_cr("Pointer is %p", p);
>>   }
>> 
>> 
>> 
>> vmmap shows:
>> 
>> 
>> VM_ALLOCATE                 10ccb4000-14ccb4000    [  1.0G     0K     0K     0K] ---/rwx SM=NUL  
>> 
>> 
>> so, looks good. 1GB, with all protection flags cleared. But System.map shows nothing for this address range.
>> 
>> 
>> Now, I commit the second half of the range:
>> 
>> 
>> 
>>   if (UseNewCode) {
>>     char* p = os::reserve_memory(G, false, mtInternal);
>>     tty->print_cr("Pointer is %p", p);
>>     bool b = os::commit_memory(p + (512 * M), 512 * M, false);
>>     assert(b,"???");
>>   }
>> 
>> 
>> vmmap shows only the committed part now, omitting the still uncommitted first half. But it gets the protection flags right again (rw now):
>> 
>> 
>> VM_ALLOCATE (reserved)      148000000-168000000    [512.0M     0K     0K     0K] rw-/rwx SM=NUL          reserved VM address space (unallocated)
>> 
>> 
>> System.map shows nothing.
>> 
>> 
>> What goes on? Is the OS lying to us? Do we have an error? Both vmmap and System.map seem to struggle, with vmmap being somewhat more correct.
>
>> @tstuefe I've look into your test, and I will modify the PR to display these regions - it was incorrectly identifying them as "free". As to the strange vmmap behaviour, I found that the two sections appeared in different places: the uncommitted spaces appeared in "==== Non-writable regions for process": `VM_ALLOCATE 300000000-320000000 [512.0M 0K 0K 0K] ---/rwx SM=NUL ` and the committed spaces in "==== Writable regions for process": `VM_ALLOCATE (reserved) 320000000-340000000 [512.0M 0K 0K 0K] rw-/rwx SM=NUL reserved VM address space (unallocated) ` I have made a few changes, track reserved and committed memory better, and uploaded an updated sample output. [vm_memory_map_89174.txt](https://github.com/user-attachments/files/18013640/vm_memory_map_89174.txt)
> 
> Yes, this is better.
> 
> Metaspace sections look like this:
> 
> 
> 0x000130000000-0x000130010000                65536 rw-/rwx pvt            0  META 
> 0x000130010000-0x000130020000                65536 rw-/rwx pvt            0  META 
> 0x000130020000-0x000130400000              4063232 ---/rwx ---      0x20000  META 
> 0x000130400000-0x000130410000                65536 rw-/rwx pvt            0  META 
> 0x000130410000-0x000134000000             62849024 ---/rwx ---     0x410000  META 
> 
> 
> A single 64MB space node. First three entries together are the initial 4MB chunk the boot class loader uses. Forth line, together with some space from the fifth line will belong to the next chunk of the next class loader.
> 
> Class space is still a bit weird:
> 
> 
> 0x018001000000-0x018001010000                65536 rw-/rwx pvt            0  CLASS 
> 0x018001010000-0x018001040000               196608 ---/rwx ---    0x1010000  CLASS 
> 0x018001040000-0x018001050000                65536 rw-/rwx pvt            0  CLASS 
> 0x018001050000-0x018008000000            117112832 ---/rwx ---    0x1050000  CLASS 
> 0x018008000000-0x018010000000            134217728 ---/rwx ---            0  CLASS 
> 0x018010000000-0x018018000000            134217728 ---/rwx ---            0  CLASS 
> 0x018018000000-0x018020000000            134217728 ---/rwx ---            0  CLASS 
> 0x018020000000-0x018028000000            134217728 ---/rwx ---            0  CLASS 
> 0x018028000000-0x018030000000            134217728 ---/rwx ---            0  CLASS 
> 0x018030000000-0x018038000000            134217728 ---/rwx ---            0  CLASS 
> 0x018038000000-0x018040000000            134217728 ---/rwx ---            0  CLASS 
> 0x018040000000-0x018041000000             16777216 ---/rwx ---       ...

> @tstuefe I ran an experiment with raw mmap, and there's no way to differentiate between one large allocation of 5*128MB and 5 smaller allocations of 128MB. I _could_ add code to fold these, but we risk loosing information.

What information would we loose?

As it is now, the display is somewhat confusing. We did not allocate the heap with multiple mmap calls, each one of 128MB in size; we use a single mmap call.

If you want to close the work for now and leave this glitch for later, we can do this too.

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PR Comment: https://git.openjdk.org/jdk/pull/20953#issuecomment-2520776514