Update comments in emmalloc.c regarding root regions and sbrk(). (#20784)

Author: juj

system/lib/emmalloc.c

@@ -11,7 +11,7 @@
  *
  *  - sbrk() is used to claim new memory (sbrk handles geometric/linear
  *  - overallocation growth)
- *  - sbrk() can be used by other code outside emmalloc.
+ *  - sbrk() can also be called by other code, not reserved to emmalloc only.
  *  - sbrk() is very fast in most cases (internal wasm call).
  *  - sbrk() returns pointers with an alignment of alignof(max_align_t)
  *
@@ -27,6 +27,8 @@
  *    merged.
  *  - Memory allocation takes constant time, unless the alloc needs to sbrk()
  *    or memory is very close to being exhausted.
+ *  - Free and used regions are managed inside "root regions", which are slabs
+ *    of memory acquired via calls to sbrk().
  *
  * Debugging:
  *
@@ -486,10 +488,11 @@ static bool claim_more_memory(size_t numBytes)
   validate_memory_regions();
 #endif
 
-  // Make sure we send sbrk requests of aligned sizes. If we do not do that then
-  // it will not return contiguous regions, which leads to use creating more
-  // root regions below, inefficiently. (Note that we assume our alignment is
-  // identical to sbrk's, see the assumptions at the start of this file.)
+  // Make sure we always send sbrk requests with the same alignment that sbrk()
+  // allocates memory at. Otherwise we will not properly interpret returned memory
+  // to form a seamlessly contiguous region with earlier root regions, which would
+  // lead to inefficiently treating the sbrk()ed region to be a new disjoint root
+  // region.
   numBytes = (size_t)ALIGN_UP(numBytes, MALLOC_ALIGNMENT);
 
   // Claim memory via sbrk
@@ -517,7 +520,7 @@ static bool claim_more_memory(size_t numBytes)
   if (startPtr == previousSbrkEndAddress)
   {
 #ifdef EMMALLOC_VERBOSE
-    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: expanding previous'));
+    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: sbrk() returned a region contiguous to last root region, expanding the existing root region'));
 #endif
     Region *prevEndSentinel = prev_region((Region*)startPtr);
     assert(debug_region_is_consistent(prevEndSentinel));
@@ -544,9 +547,11 @@ static bool claim_more_memory(size_t numBytes)
   }
   else
   {
-    // Create a root region at the start of the heap block
+    // Unfortunately some other user has sbrk()ed to acquire a slab of memory for themselves, and now the sbrk()ed
+    // memory we got is not contiguous with our previous managed root regions.
+    // So create a new root region at the start of the sbrk()ed heap block.
 #ifdef EMMALLOC_VERBOSE
-    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: creating new root region'));
+    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: sbrk() returned a disjoint region to last root region, some external code must have sbrk()ed outside emmalloc(). Creating a new root region'));
 #endif
     create_used_region(startPtr, sizeof(Region));