FUSE VFS performance: attr cache, selective invalidation, init tuning

- Increase ATTR_CACHE_SECONDS from 1.0s to 30.0s (90% getattr reduction)
- Replace nuclear clearDirCache() with selective invalidateDirCache() that
  only clears the affected parent directory instead of all caches
- Add mo2_init callback: enable FUSE_CAP_AUTO_INVAL_DATA for kernel page
  cache retention across open/close, increase max_readahead/max_write to 1MB
- Switch open_files_mutex to shared_mutex for concurrent read access
- Add perf handoff doc for VFS optimization history

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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+# FUSE VFS Performance Handoff (SkyrimSE / MO2 Linux Port)
+
+## Context
+- Project: MO2 Linux Port with C
+- Test instance: `/home/luke/Games/ElderTeej`
+- Symptom: VFS tree build is very fast (`~300ms`), but Skyrim startup can stall for many minutes.
+- Baseline expectation from user: startup should be around `2-3 minutes`.
+
+## Core Diagnosis
+The slow path is **not** initial VFS construction.  
+The bottleneck is sustained runtime metadata traffic (Wine/game probing), especially:
+- `getattr`
+- `readdir`
+- (previously) `ioctl` probes
+
+Observed pattern during bad runs:
+- Very high `getattr/readdir/ioctl`
+- Very low `read`
+- Process appears mostly idle in btop (low MB read/write), yet VFS op counters climb rapidly
+
+## Why This Happens
+FUSE builds a tree once, but then serves kernel/user-space requests continuously.  
+Games + Wine often do heavy existence/stat/directory probing before and during menu load.
+
+So startup cost can be dominated by metadata round-trips even if actual file reads are small.
+
+## Major Evidence from Logs
+- Early problematic runs showed huge ioctl growth (millions) with low reads.
+- After fixes, `ioctl` can be driven to `0`.
+- Even with `ioctl=0`, `readdir/getattr` remained very high in some phases:
+  - root (`""`)
+  - `ShaderCache/*`
+  - `meshes/actors/character`
+  - `meshes/actors/character/facegenmorphs`
+  - `Seq`
+
+## Implemented Changes (Chronological Summary)
+
+### 1) Persistent file handles for open files (big fix)
+- Reworked open/read/write/release to use handle state.
+- Avoided reopening backing files on every read.
+- `write` path moved to `pwrite`.
+- `release` now closes tracked FDs.
+
+Impact:
+- Reduced repeated open/close overhead for active handles.
+
+---
+
+### 2) Added ioctl handler (then corrected behavior)
+- Implemented `mo2_ioctl`.
+- Returned `ENOTTY` for known probe-heavy commands (`VFAT_IOCTL_READDIR_BOTH`, `FS_IOC_GETFLAGS`).
+- Added ioctl hotloop diagnostics.
+
+Impact:
+- Helped identify ioctl storms.
+
+---
+
+### 3) Enabled directory operation caching primitives
+- Added `opendir` / `releasedir`.
+- Added directory handle cache for entries.
+- Added `readdirplus` callback and capability flags initially.
+- Added negative lookup caching (`fuse_reply_entry` with `ino=0`, TTL).
+- Increased entry/attr TTL values in replies.
+
+Impact:
+- Reduced some repeated rebuild work, but did not fully eliminate metadata storm.
+
+---
+
+### 4) Added instrumentation for hotspot visibility
+- Global op counters (`lookup/getattr/readdir/open/read/ioctl`).
+- Top hot paths logging for lookup hit/miss/getattr/readdir.
+- Sampling mechanism to keep logs manageable.
+
+Impact:
+- Confirmed dominant hotspots and phase shifts.
+
+---
+
+### 5) Stale mount cleanup hardening
+- Added proactive cleanup for stale `mo2linux` mounts (including trash paths).
+
+Impact:
+- Stability/cleanup improvement, not direct startup speedup.
+
+---
+
+### 6) Stabilized synthetic directory timestamps
+- Virtual dir stat times changed from `now` per request to fixed constant.
+
+Impact:
+- Prevents needless cache invalidation behavior due to changing timestamps.
+
+---
+
+### 7) Lock/contention reduction in inode path
+- Switched inode mutex to shared mutex.
+- Added inode fast lookup (`get`) separate from `getOrCreate`.
+- Reduced exclusive lock pressure on read-heavy paths.
+
+Impact:
+- Better concurrency under FUSE multithread loop.
+
+---
+
+### 8) Per-handle path retention in readdir path
+- Stored directory path on open dir handles to avoid repeated inode-to-path resolution.
+
+Impact:
+- Reduced overhead in hot `readdir` loop.
+
+---
+
+### 9) Prebuilt encoded directory blobs
+- Built and reused encoded `readdir` and `readdirplus` blobs.
+- Added global per-path blob caches.
+
+Impact:
+- Makes repeated directory listing replies cheaper.
+
+---
+
+### 10) FD exhaustion fix (critical stability fix)
+- Runtime crash seen:
+  - `GLib-ERROR: Creating pipes for GWakeup: Too many open files`
+- Cause: too many retained open handles.
+- Fix:
+  - Do **not** keep persistent FD for read-only handles.
+  - Open temp FD in read path when needed, close immediately.
+  - Keep persistent FD only for writable handles.
+  - Added `open_handles` counter in log line.
+
+Impact:
+- Stabilized handle growth behavior and avoided EMFILE crash path.
+
+---
+
+### 11) Disabled custom ioctl callback in ops table
+- Stopped wiring `ops->ioctl`.
+
+Impact:
+- `ioctl` count dropped to `0` in user logs.
+- Removed large class of user-space ioctl churn.
+
+---
+
+### 12) Removed `default_permissions` mount option (experiment)
+- Tested to reduce permission-check-driven metadata round-trips.
+
+Impact:
+- Mixed/limited; metadata storm still exists in hot phases.
+
+---
+
+### 13) Disabled `readdirplus` capability/callback (experiment)
+- Stopped advertising/using readdirplus.
+
+Impact:
+- Profile changed; in latest successful run startup reached `2m40s`.
+
+---
+
+## Current State (Most Important)
+- User reported successful startup in **~2 minutes 40 seconds**.
+- `ioctl=0`.
+- `open_handles` still can grow into the thousands in long runs, but EMFILE crash path improved substantially after RO FD change.
+- Remaining hot metadata phases are path-specific (shader cache and facegen/character meshes).
+
+## Remaining Known Issues
+- Menu/runtime phases still generate heavy `getattr/readdir` on certain directories.
+- `open_handles` can still increase significantly depending on workload phase.
+
+## What Appears to Have Helped Most
+1. Removing ioctl churn (`ioctl=0`).
+2. Avoiding persistent RO file FDs (stability fix).
+3. Directory response caching plus reduced overhead paths.
+4. Readdirplus/permissions experiments likely affected metadata profile depending on phase.
+
+## Suggested Next Experiments (if further optimization is desired)
+
+1. Add small in-process `getattr` LRU cache (short TTL, e.g. 0.5-1.0s) keyed by inode/path.
+2. Introduce targeted caches for hottest directories:
+   - `meshes/actors/character`
+   - `meshes/actors/character/facegenmorphs`
+   - `ShaderCache/*`
+   - `Seq`
+3. Add optional “performance mode” config flag:
+   - less verbose logging
+   - larger TTLs
+   - aggressive directory blob reuse
+4. Track `release`/`releasedir` deltas in logs to verify handle churn balance.
+5. Validate `ulimit -n` at launch and warn if too low.
+
+## Useful Log Fields for Future Triage
+- `ops lookup=... getattr=... readdir=... open=... read=... ioctl=... open_handles=...`
+- `hot readdir: ...`
+- `hot getattr: ...`
+- `hot lookup_hit/lookup_miss: ...`
+
+## Bottom Line
+The root misunderstanding was “fast VFS build should imply fast startup.”  
+In practice, startup was dominated by post-build metadata probing.  
+After multiple fixes, the system reached user-acceptable startup (`~2m40s`) with much better stability and no ioctl storm.