p12-log: Add write-ahead logging with file layer transaction support

[Yash-Rawat-IIT-D]

Overview

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Integrated p12-log write-ahead logging functionality from PR #8 with comprehensive transaction wrappers and file layer improvements. Built on p11-file-layer (PR #21).

Credit: Original p12-log implementation by Amber Agarwal (Amber-Agarwal, PR #8)
This PR: Integration work including mkdir() implementation, begin_op/end_op wrappers in file.rs, log_write() integration, and welcome() rewrite to match C version

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Changes Made (Integration Work)

1. File Layer Transaction Wrappers

File: src/file.rs

• Added begin_op()/end_op() wrappers to all filesystem-modifying functions:
  • fileclose() - Wraps iput with transaction (lines 111-113)
  • filewrite() - Wraps write loop with transaction (lines 191-198)
  • unlink() - Begin at start, end_op before all 4 return points (lines 242-302)
  • open() - Begin at start, end_op before all 5 return points (lines 348-390)
  • mkdir() - Complete begin_op/end_op wrapper (lines 399-411)
• Changed create() visibility from pub fn to fn (private, line 306)

Ensures all multi-step filesystem operations are atomic and crash-consistent. Matches C file.c transaction patterns exactly.

2. Logging Integration in Filesystem Layer

File: src/fs.rs

• Replaced all bwrite() calls with log::log_write() in 7 locations:
  • bzero() - Zero disk blocks (line 208)
  • balloc() - Block allocation (line 234)
  • bfree() - Block deallocation (line 264)
  • ialloc() - Inode allocation (line 287)
  • iupdate() - Inode updates (line 385)
  • bmap() - Block mapping with allocation (line 488)
  • writei() - File writing (line 581)

Write-ahead logging requires all disk modifications go through log_write() for crash recovery. Direct bwrite() bypasses logging system.

3. mkdir() Implementation

File: src/file.rs

• Implemented mkdir() function (lines 399-411)
• Creates directory with . and .. entries
• Proper transaction wrapping with begin_op/end_op
• Matches C sys_mkdir() semantics

Missing from original p11 implementation, required for complete file layer functionality.

4. welcome() Function Rewrite

File: src/lib.rs

• Completely rewrote welcome() to match C main.c version exactly (lines 55-94)
• Uses file layer primitives: mkdir(), open(), filewrite(), fileread(), unlink()
• Replaces low-level filesystem operations with high-level file descriptor API
• Tests complete file lifecycle: create → write → read → delete

Demonstrates proper usage of file layer with transaction system. Matches C testing approach.

5. Log System Initialization

File: src/lib.rs

• Added log::initlog(param::ROOTDEV) call during kernel initialization (line 114)
• Recovers from log on startup for crash consistency

Critical for write-ahead logging - must recover incomplete transactions before filesystem use.

File: src/param.rs

• Added LOGSIZE = 30 (MAXOPBLOCKS * 3)
• Added MAXOPBLOCKS = 10
• Added NFILE = 100

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Expected Behavior

On boot:

• Initializes logging system and recovers from any incomplete transactions
• Creates /foo directory
• Creates /foo/greet.txt with content "Hello from file layer!"
• Reads and prints file contents
• Deletes /foo/greet.txt
• Verifies /foo is empty
• All operations are crash-consistent via write-ahead logging

Testing

Testing Environment: WSL (Windows Subsystem for Linux)

[PASS] Build: make kernel completes with 0 warnings, 0 errors
[PASS] Build: make xv6.img generates filesystem image with LOGSIZE=30
[PASS] Runtime: make qemu shows expected behavior:
       - Log initialized: Reads superblock, recovers from log
       - Directory creation: /foo created successfully
       - File operations: /foo/greet.txt create → write → read → delete cycle
       - Transaction atomicity: All multi-step operations complete or rollback
       - Crash consistency: Log replay works correctly on restart

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Files Modified

• src/log.rs - Complete write-ahead logging implementation
• src/file.rs - Transaction wrappers, mkdir() implementation
• src/fs.rs - All bwrite() → log_write() conversions
• src/lib.rs - welcome() rewrite, initlog() call
• src/param.rs - Log system constants

Constraints Met

• Semantic equivalence with C implementation
• Zero compilation warnings/errors
• Crash-consistent filesystem operations
• Transaction boundaries match C exactly

[KoRaghav]

Please import the crate at the top instead

[KoRaghav]

Merged manually