Flushes hash cache data file's mmap after done writing (#2567)

(cherry picked from commit 65f6466)

accounts-db/src/cache_hash_data.rs

@@ -5,7 +5,7 @@ use {
     crate::{accounts_hash::CalculateHashIntermediate, cache_hash_data_stats::CacheHashDataStats},
     bytemuck_derive::{Pod, Zeroable},
     memmap2::MmapMut,
-    solana_measure::measure::Measure,
+    solana_measure::{measure::Measure, measure_us},
     solana_sdk::clock::Slot,
     std::{
         collections::HashSet,
@@ -371,10 +371,17 @@ impl CacheHashData {
         });
         assert_eq!(i, entries);
         m2.stop();
+        // We must flush the mmap after writing, since we're about to turn around and load it for
+        // reading *not* via the mmap.  If the mmap is never flushed to disk, it is possible the
+        // entries will *not* be visible when the reader comes along.
+        let (_, measure_flush_us) = measure_us!(cache_file.mmap.flush()?);
+        m.stop();
         self.stats
             .write_to_mmap_us
             .fetch_add(m2.as_us(), Ordering::Relaxed);
-        m.stop();
+        self.stats
+            .flush_mmap_us
+            .fetch_add(measure_flush_us, Ordering::Relaxed);
         self.stats.save_us.fetch_add(m.as_us(), Ordering::Relaxed);
         self.stats.saved_to_cache.fetch_add(1, Ordering::Relaxed);
         Ok(())

accounts-db/src/cache_hash_data_stats.rs

@@ -11,6 +11,7 @@ pub struct CacheHashDataStats {
     pub save_us: AtomicU64,
     pub saved_to_cache: AtomicUsize,
     pub write_to_mmap_us: AtomicU64,
+    pub flush_mmap_us: AtomicU64,
     pub create_save_us: AtomicU64,
     pub load_us: AtomicU64,
     pub read_us: AtomicU64,
@@ -61,6 +62,11 @@ impl CacheHashDataStats {
                 self.write_to_mmap_us.load(Ordering::Relaxed),
                 i64
             ),
+            (
+                "flush_mmap_us",
+                self.flush_mmap_us.load(Ordering::Relaxed),
+                i64
+            ),
             (
                 "create_save_us",
                 self.create_save_us.load(Ordering::Relaxed),