RFR: 8253683: Clean up and clarify uses of os::vm_allocation_granularity

[Frederic Thevenet]

On Thu, 4 Dec 2025 10:03:05 GMT, Frederic Thevenet <fthevenet at openjdk.org> wrote:

>> Hi everyone,
>> 
>> `os::vm_allocation_granularity()` is meant to describe the alignment restrictions of the operating system when we reserve memory. That is 64 KiB on Windows (`VirtualAlloc`) and 256 MiB on AIX (with `shmat`). On every other platform it happens to match the page size. The page size (available via `os::vm_page_size()`) is what matters when we later commit or protect the reserved pages.
>> 
>> Because the functions are poorly documented and the two numbers are identical on most systems, they have gradually been used more and more interchangeably. We now have many code paths that round **sizes** up to `os::vm_allocation_granularity()` or assert that a size is a multiple of it. That is wrong. Only addresses need that alignment, sizes merely have to be page-aligned. Places that round sizes should instead use `os::vm_page_size()` as they are unrelated to attach alignment.
>> 
>> For this change I have gone over the call sites of `os::vm_allocation_granularity()` and where it was being used to pad or sanity-check a size I have instead replaced it with `os::vm_page_size()`. The calls that genuinely deal with an attach address are left untouched.
>> 
>> Testing:
>> - Oracle tiers 1-8
>
> src/hotspot/share/jfr/recorder/storage/jfrVirtualMemory.cpp line 117:
> 
>> 115:   assert(_rs.size() != 0, "invariant");
>> 116:   assert(is_aligned(_rs.base(), os::vm_allocation_granularity()), "invariant");
>> 117:   assert(is_aligned(_rs.size(), os::vm_page_size()), "invariant");
> 
> The assert at line 117 is now redundant with the one done at 125.
> AFAICT, neither `os::trace_page_sizes` nor `MemTracker::record_virtual_memory_tag`  have side effects that could affect `_rs.size()`, so line 125 could probably be removed.

Although this is not a change made by this PR, I'm a bit puzzled by the need to first assert `_rs.base()` is aligned on `vm_allocation_granularity` at 116 and then on `vm_page_size` at 124; is this needed? Should we attempt to maybe clarify that as part of this PR?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28493#discussion_r2588357427