Fix commit accounting for large pages

[cshung]

This change fixes an integer underflow that happens in the commit accounting, here is the sequence of events that will lead to it.

Under use_large_page_p, we created a new heap segment, we passed SEGMENT_INITIAL_COMMIT (which is just a page) to virtual_commit, so committed_by_oh[0] is increased by SEGMENT_INITIAL_COMMIT.

Later, the heap_segment_committed value for the segment is changed to size, which is the full region size.

Eventually, we return_free_region, which will subtract committed_by_oh[0] by the full region size, which will lead to a subtraction integer underflow.

This will result in a huge number, and that will fail future virtual_commit calls, leading to unexpected OOM.

The fix simply makes sure we add the full region size to the committed_by_oh[0] to begin with.

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Issue Details

---

This change fixes an integer underflow that happens in the commit accounting, here is the sequence of events that will lead to it.

Under use_large_page_p, we created a new heap segment, we passed SEGMENT_INITIAL_COMMIT (which is just a page) to virtual_commit, so committed_by_oh[0] is increased by SEGMENT_INITIAL_COMMIT.

Later, the heap_segment_committed value for the segment is changed to size, which is the full region size.

Eventually, we return_free_region, which will subtract committed_by_oh[0] by the full region size, which will lead to a subtraction integer underflow.

This will result in a huge number, and that will fail future virtual_commit calls, leading to unexpected OOM.

The fix simply makes sure we add the full region size to the committed_by_oh[0] to begin with.

Author:	cshung
Assignees:	cshung
Labels:	area-GC-coreclr
Milestone:	-

[cshung]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3498854226