Raise error if ucx detects VRAM mem as host mem

[x41lakazam]

Summary

In the case where UCX is built without CUDA, ucp_mem_map silently succeeds on GPU pointers but misclassify them as host memory.
Previously, nixlUcxContext::memReg only logged a warning in this case and returned success.
This caused agent's registerMem, making the UCX backend appear fully functional for VRAM — but actual transfers would fail much later in prepXferDlist, makeXferReq, or during the transfer itself, producing confusing errors far from the root cause.

This change promotes the warning to an error and makes memReg fail immediately when VRAM is detected as host memory by UCX. The memory mapping is properly cleaned up (ucp_mem_unmap) before returning.

Without this fix, users with a UCX installation lacking CUDA support see the following behavior:
registerMem(VRAM_SEG, ...) succeeds (with only a log warning).
The backend appears available for GPU transfers.
Transfer preparation or execution fails with an unrelated error, making the root cause (missing UCX CUDA support) very hard to diagnose.

With this fix, registerMem fails immediately with a clear error message pointing to the missing UCX CUDA support, allowing callers to gracefully fall back or report the issue at the right time.

Summary by CodeRabbit

Bug Fixes

• Enhanced error detection and resource cleanup for GPU memory registration. The system now validates configurations more strictly and immediately terminates registration with proper resource cleanup when issues are detected, preventing continuation with problematic setups.

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ℹ️ Recent review info

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Plan: Pro

Run ID: bc0fca59-380a-4d71-9da9-882773c1249a

📥 Commits

Reviewing files that changed from the base of the PR and between ae80d8d and bd4219e.

📒 Files selected for processing (1)

• src/plugins/ucx/ucx_utils.cpp

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📝 Walkthrough

Walkthrough

Modified error handling in nixlUcxContext::memReg to treat VRAM memory reported as HOST by UCX as a hard error. When this condition occurs, the function now logs an error, unmaps allocated memory, and returns -1 instead of issuing a warning and continuing.

Changes

Cohort / File(s)	Summary
VRAM Registration Error Handling src/plugins/ucx/ucx_utils.cpp	Enhanced error handling in memReg to convert a UCX HOST classification of VRAM memory into a fatal error with cleanup, replacing the previous warning path with immediate unmapping and -1 return code.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Fail registerMem when VRAM is misclassified as host memory #1385: Implements the identical fix in nixlUcxContext::memReg to treat UCX-classified HOST memory as VRAM error with logging, unmapping, and -1 return instead of warning.

Suggested reviewers

• yosefe
• brminich
• gleon99

Poem

🐰 A VRAM mixed up as HOST, what a plight,
Now caught with an error, handled just right!
We unmap and return with a swift -1,
No more warnings whispered—the fix is done! 🎯

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: raising an error when UCX detects VRAM as host memory, matching the core objective of the PR.
Description check	✅ Passed	The description comprehensively covers what the change does, why it's needed (with detailed context about the problem), and the before/after behavior, aligning well with the template structure.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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