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

[Simon Tooke]

On Mon, 2 Dec 2024 14:52:38 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> 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?
>> 
>> I think there is only one JAVAHEAP segment because due to an issue with my build[1] there was no CDS archive available.
>> 
>> I will look at the META and CLASS entries and see if there are hidden properties that I can surface, or if there's another reason for so many entries.
>> 
>> [1] I had given a target architecture to the configure command, which turned on cross-compiles (which disables CDS archive building) even when building on the target platform.
>
> 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)

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PR Comment: https://git.openjdk.org/jdk/pull/20953#issuecomment-2518444621