MAP_SHARED even for read-only

[pknowles]

MAP_PRIVATE is backed by swap, which limits the maximum size

Summary by CodeRabbit

• Refactor
  • Updated memory mapping behavior to ensure consistent file access patterns across all usage scenarios. No changes to the public API.

[coderabbitai]

Walkthrough

Modified the MappedFile constructor in the Linux implementation to consistently use MAP_SHARED for memory mapping, replacing the previous conditional behavior that selected MAP_PRIVATE for non-writable mappings and MAP_SHARED for writable ones.

Changes

Cohort / File(s)	Summary
MappedFile Linux Mapping Behavior include/decodeless/detail/mappedfile_linux.hpp	Changed constructor memory mapping flags from conditional (MAP_PRIVATE for read-only, MAP_SHARED for writable) to always use MAP_SHARED

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Fix compilation on Linux, macOS and Windows. #10: Also modifies the same file to update mapping flags, suggesting an ongoing refinement of the memory mapping strategy.

Poem

🐰 A rabbit hops through memory lanes,
Where MAP_SHARED now reigns,
No more split decisions, hooray!
All mappings share the same way. 🗺️

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The pull request title "MAP_SHARED even for read-only" directly and accurately describes the primary change in the changeset. The title clearly communicates that the modified code now uses MAP_SHARED memory mapping flags universally, including for read-only file mappings, which were previously handled with MAP_PRIVATE. The title is concise, specific, and would allow a developer reviewing the repository history to immediately understand the technical nature and intent of the change. It avoids vague terminology and noise while capturing the essence of the modification.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch dev-shared-readonly

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: ASSERTIVE

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between d2b9b43 and a214250.

📒 Files selected for processing (1)

• include/decodeless/detail/mappedfile_linux.hpp (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (2)

• GitHub Check: build (windows-latest, Release, cl)
• GitHub Check: build (windows-latest, Release, cl)

🔇 Additional comments (1)

include/decodeless/detail/mappedfile_linux.hpp (1)

199-199: Semantic change noted, but verify file immutability assumptions are documented.

Changing from MAP_PRIVATE to MAP_SHARED for read-only mappings does alter semantics—the application will now see live file changes rather than a stable snapshot. However, this is unlikely to be problematic in practice:

• Read-only mode (opened with O_RDONLY, protected PROT_READ) prevents the application from writing changes.
• The codebase shows no evidence of concurrent file modification scenarios; usage is static and single-threaded.
• MAP_SHARED for read-only is a standard approach to avoid swap-backed limitations.

Action: Document the assumption that mapped files remain immutable (or at least don't change unexpectedly during the application's use of the mapping). Add a brief note in the file type documentation clarifying this contract, especially if this is a public API.

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