page cache: small code cleanups (#5125)

## Problem

I saw these things while working on #5111.

## Summary of changes

* Add a comment explaining why we use `Vec::leak` instead of
`Vec::into_boxed_slice` plus `Box::leak`.
* Add another comment explaining what `valid` is doing, it wasn't very
clear before.
* Add a function `set_usage_count` to not set it directly.

pageserver/src/page_cache.rs

@@ -203,6 +203,11 @@ impl Slot {
             Err(usage_count) => usage_count,
         }
     }
+
+    /// Sets the usage count to a specific value.
+    fn set_usage_count(&self, count: u8) {
+        self.usage_count.store(count, Ordering::Relaxed);
+    }
 }
 
 pub struct PageCache {
@@ -263,6 +268,7 @@ pub struct PageWriteGuard<'i> {
     inner: RwLockWriteGuard<'i, SlotInner>,
 
     // Are the page contents currently valid?
+    // Used to mark pages as invalid that are assigned but not yet filled with data.
     valid: bool,
 }
 
@@ -535,7 +541,7 @@ impl PageCache {
             // Make the slot ready
             let slot = &self.slots[slot_idx];
             inner.key = Some(cache_key.clone());
-            slot.usage_count.store(1, Ordering::Relaxed);
+            slot.set_usage_count(1);
 
             return Ok(ReadBufResult::NotFound(PageWriteGuard {
                 inner,
@@ -596,7 +602,7 @@ impl PageCache {
             // Make the slot ready
             let slot = &self.slots[slot_idx];
             inner.key = Some(cache_key.clone());
-            slot.usage_count.store(1, Ordering::Relaxed);
+            slot.set_usage_count(1);
 
             return Ok(WriteBufResult::NotFound(PageWriteGuard {
                 inner,
@@ -795,6 +801,8 @@ impl PageCache {
     fn new(num_pages: usize) -> Self {
         assert!(num_pages > 0, "page cache size must be > 0");
 
+        // We use Box::leak here and into_boxed_slice to avoid leaking uninitialized
+        // memory that Vec's might contain.
         let page_buffer = Box::leak(vec![0u8; num_pages * PAGE_SZ].into_boxed_slice());
 
         let size_metrics = &crate::metrics::PAGE_CACHE_SIZE;