Fix GC page size assumptions

[kkokosa]

Fixes #133743.

On systems with OS pages larger than 4 KiB, GC regions could have mark bitmaps smaller than one OS page, causing regions to share committed bitmap pages and corrupt commitment accounting. The pre-OOM path also compared region commitment against the logical 4 KiB GC_PAGE_SIZE instead of the platform initial commit size.

This change:

• derives the minimum safe region size from OS_PAGE_SIZE and mark-word coverage
• rejects explicitly configured region sizes below that minimum
• uses SEGMENT_INITIAL_COMMIT for pre-OOM reclamation
• adds a background-GC regression test

Validated on ARM64 Linux with both 64 KiB and 4 KiB kernels. The regression fails before and passes after the fix on 64 KiB pages, while regular 4 KiB region sizing remains unchanged.

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🟡 Changes recommended

Existing ARM64 configurations may fail with 64-KiB pages, and the sizing comment remains inaccurate.

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Pull request overview

Fixes GC region sizing and commitment accounting on systems with OS pages larger than 4 KiB.

Changes:

• Enforces a page-size-aware minimum GC region size.
• Uses SEGMENT_INITIAL_COMMIT for pre-OOM reclamation.
• Adds regression coverage and updates GC error resources.

File summaries

File	Summary
src/tests/GC/Regressions/Github/Runtime_133743/Runtime_133743.csproj	Configures the regression test.
src/tests/GC/Regressions/Github/Runtime_133743/Runtime_133743.cs	Adds background-GC regression coverage.
src/coreclr/pal/prebuilt/corerror/mscorurt.rc	Updates the corresponding error resource.
src/coreclr/inc/corerror.xml	Updates GC region-size error text.
src/coreclr/gc/regions_segments.cpp	Corrects initial-commit comparison.
src/coreclr/gc/interface.cpp	Validates minimum region size. Moderate issue (3 votes): existing ARM64 configurations may fail on 64-KiB-page systems; nit (1 vote): the sizing comment no longer matches behavior.
src/coreclr/gc/gcpriv.h	Documents logical GC page granularity.

Review details

Suppressed comments (1)

src/coreclr/gc/interface.cpp:451

• This revised comment still says automatic sizing chooses only 4, 2, or 1 MiB, but the max below can now select the platform-dependent minimum (8 MiB on 64-KiB pages). Please document the base-size selection and the subsequent minimum-size adjustment so the comment matches the new behavior.

    // except for the smallest case.

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[kkokosa]

Should we use something like this, fe. for Runtime_126043, to exclude those tests on non-4 KiB machines?

<CLRTestBashPreCommands><![CDATA[
  $(CLRTestBashPreCommands)
  page_size=`getconf PAGESIZE`
  if [ "$page_size" -gt 32768 ]; then
    echo "Test requires 4 MiB GC regions, which are unsupported with ${page_size}-byte OS pages."
    exit 0
  fi
  export DOTNET_gcServer=1
  export DOTNET_GCHeapCount=1
  export DOTNET_GCRegionSize=400000
]]></CLRTestBashPreCommands>

[jkotas]

cc @tmds

[janvorli]

I would rather say:

Suggested change

	<Comment>During a GC initialization, the GC Region Size configuration is invalid.</Comment>
	<Comment>During a GC initialization, the GC Region Size configuration must be valid.</Comment>