RFR: 8342504: Remove NMT header and footer canaries

[Gerard Ziemski]

On Thu, 17 Oct 2024 10:04:16 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> Today NMT has two canaries: A header and a footer canary. These enable mainly two things:
> 
> 1. For NMT to aid in describing a pointer
> 2. A basic form of out-of-bounds protection
> 
> With the introduction of UBSan and Asan into OpenJDK we have gained stronger tools for this sort of analysis, without requiring NMT to be activated. Therefore, I believe that point 2 is no longer something that NMT needs to support. For point number one, we will unfortunately be losing this ability.
> 
> I want to delete these canaries to open up a few free bytes. These can allow us to have "practically unlimited" (4 bytes) of memory tags.
> 
> tier1-tier2 tests succeeded.
> 
> I am awaiting discussion on the Hotspot-dev mailing list, but keeping this PR open for review.

I see code removal, tweaks to malloc header size, tests fixes, so all looks good, except for my question about `MallocTracker::print_pointer_information()`

src/hotspot/share/nmt/mallocTracker.cpp line 227:

> 225: // totally failproof and may give a wrong answer. It is safe in that it will never
> 226: // crash, even when encountering unmapped memory.
> 227: bool MallocTracker::print_pointer_information(const void* p, outputStream* st) {

It looks like the only thing that had to go was `candidate->looks_valid()`

Isn't it useful to keep the rest of this code though?

-------------

Changes requested by gziemski (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21560#pullrequestreview-2389125128
PR Review Comment: https://git.openjdk.org/jdk/pull/21560#discussion_r1812997770