From 3cdb7be478040e799ca878af2d00180e2b7c5316 Mon Sep 17 00:00:00 2001
From: Nathaniel Wesley Filardo <nfilardo@microsoft.com>
Date: Sun, 17 Oct 2021 18:12:48 +0100
Subject: [PATCH] MetaEntry: split get_metaslab by intent

---
 src/backend/address_space_core.h |  2 +-
 src/mem/localalloc.h             |  4 ++--
 src/mem/metaslab.h               | 16 ++++++++++++++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/backend/address_space_core.h b/src/backend/address_space_core.h
index dcbca96..f8b7251 100644
--- a/src/backend/address_space_core.h
+++ b/src/backend/address_space_core.h
@@ -125,7 +125,7 @@ namespace snmalloc
         const MetaEntry& t = Pagemap::template get_metaentry<false>(
           local_state, address_cast(base));
         return capptr::Chunk<FreeChunk>(
-          reinterpret_cast<FreeChunk*>(t.get_metaslab()));
+          reinterpret_cast<FreeChunk*>(t.get_metaslab_no_remote()));
       }
 
       return base->next;
diff --git a/src/mem/localalloc.h b/src/mem/localalloc.h
index db4bbfa..43423af 100644
--- a/src/mem/localalloc.h
+++ b/src/mem/localalloc.h
@@ -534,8 +534,8 @@ namespace snmalloc
         UNUSED(size);
 #  endif
 
-        ChunkRecord* slab_record =
-          reinterpret_cast<ChunkRecord*>(entry.get_metaslab());
+        auto slab_record =
+          static_cast<ChunkRecord*>(entry.get_metaslab_no_remote());
 
         SNMALLOC_ASSERT(
           address_cast(slab_record->meta_common.chunk) == address_cast(p_tame));
diff --git a/src/mem/metaslab.h b/src/mem/metaslab.h
index 1057c14..fcbb293 100644
--- a/src/mem/metaslab.h
+++ b/src/mem/metaslab.h
@@ -243,8 +243,24 @@ namespace snmalloc
         pointer_offset(reinterpret_cast<uintptr_t>(remote), sizeclass);
     }
 
+    /**
+     * Return the Metaslab field as a void*, guarded by an assert that there is
+     * no remote that owns this chunk.
+     */
+    [[nodiscard]] SNMALLOC_FAST_PATH void* get_metaslab_no_remote() const
+    {
+      SNMALLOC_ASSERT(get_remote() == nullptr);
+      return static_cast<void*>(meta);
+    }
+
+    /**
+     * Return the Metaslab metadata associated with this chunk, guarded by an
+     * assert that this chunk is being used as a slab (i.e., has an associated
+     * owning allocator).
+     */
     [[nodiscard]] SNMALLOC_FAST_PATH Metaslab* get_metaslab() const
     {
+      SNMALLOC_ASSERT(get_remote() != nullptr);
       return meta;
     }