Claude review: drm/gem-shmem: Track folio accessed/dirty status in vmap

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Review:**

This adds folio tracking for vmap operations:

**SHMEM helper changes:**
```c
@@ drm_gem_shmem_vmap_locked()
+			shmem->pages_mark_accessed_on_put = true;
+			shmem->pages_mark_dirty_on_put = true;
```

Sets flags so pages are marked when released. This is conservative - assumes all vmap'd pages are accessed and dirtied.

**Issue 1 - Overly conservative:**
The commit message states: "There's no means of handling dirty status in vmap as there's no write-only mapping available."

True, but this means ALL vmap operations mark pages dirty, even read-only ones. This could cause unnecessary writeback for buffers that are only read via vmap (e.g., reading texture data uploaded by client).

Consider:
- Adding a `vmap_readonly` flag to only set `accessed_on_put`
- Let callers specify read-only intent
- Default to current behavior for compatibility

**Issue 2 - Clearing flags:**
```c
+		shmem->pages_mark_accessed_on_put = false;
+		shmem->pages_mark_dirty_on_put = false;
```

These flags are cleared in `drm_gem_shmem_put_pages_locked()`. **But what about the imagination driver change?**

**Imagination driver change:**
```c
-	shmem_obj->pages_mark_dirty_on_put = true;  // Was in pvr_gem_object_create()
+	shmem_obj->pages_mark_dirty_on_put = true;  // Now in pvr_gem_object_free()
```

**Behavior change analysis:**

**Before:**
1. Object created → `pages_mark_dirty_on_put = true`
2. Any get/put pages cycle → marks dirty
3. Internal vmap/vunmap → marks dirty
4. Repeat...
5. Object destroyed

**After:**
1. Object created → flag not set
2. Vmap → sets `pages_mark_dirty_on_put = true`
3. Vunmap/put pages → marks dirty, **clears flag to false**
4. Next vmap → sets flag again
5. Object destroyed → sets `pages_mark_dirty_on_put = true`, then calls `drm_gem_shmem_free()`

**Issue 3 - Imagination driver race:**
At object destruction:
```c
shmem_obj->pages_mark_dirty_on_put = true;
drm_gem_shmem_free(shmem_obj);
```

If pages are still mapped at destruction (refcount > 0), setting the flag might not have the intended effect. The `drm_gem_shmem_free()` should vunmap if needed, which would trigger:
```c
drm_gem_shmem_put_pages_locked() {
    ...
    shmem->pages_mark_dirty_on_put = false;  // Clears the flag we just set!
    ...
}
```

So setting it right before free might be too late. Need to verify the exact sequence in `drm_gem_shmem_free()`.

**Issue 4 - Commit message mismatch:**
The commit says: "move the flag setting from init to cleanup. This ensures writeback of modified pages but does not interfere with the internal vmap/vunmap calls."

But how does moving to cleanup "ensure writeback of modified pages"? The flag is set, then immediately cleared by the put_pages call. The actual writeback happens during the vmap/vunmap cycles due to patch 6's changes to vmap_locked(), not because of the flag in free().

This seems like a workaround to prevent double-marking or interference, but the explanation doesn't match the code.

**Issue 5 - Missing v3d similar fix:**
Commit message: "V3d already implements this behaviour."

Should verify that v3d doesn't need similar changes. If v3d sets `pages_mark_dirty_on_put = true` at init, won't it conflict with the vmap changes that now clear the flag?

**Verdict:** ⚠ Overly conservative marking, imagination driver change needs better explanation and possible race condition, v3d compatibility unclear

---

## SUMMARY OF ISSUES

**Critical (must fix before merge):**
1. **PATCH 5**: Missing locking in `pfn_mkwrite` - race condition with page array access
2. **PATCH 6**: Imagination driver change poorly explained and potentially racy

**Major (should fix):**
3. **PATCH 3**: Error handling from PMD insertion unclear
4. **PATCH 4**: Confusing control flow with uninitialized-looking variable usage
5. **PATCH 5**: PMD write handling incomplete - defeats huge page benefits

**Minor (consider fixing):**
6. **PATCH 2**: Clarify when page can be NULL and if WARN is appropriate
7. **PATCH 5**: file_update_time() overhead on every write
8. **PATCH 6**: Overly conservative dirty marking for read-only vmap
9. **PATCH 6**: Verify v3d driver compatibility

**Testing gaps:**
10. No testing on actual hardware-accelerated drivers
11. No performance benchmarks for vmap/vunmap paths
12. No stress testing for concurrent access patterns

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Generated by Claude Code Patch Reviewer