If you try to create a client arena that's too small for the full complement of zones, return mps_res_memory instead of asserting.

If you try to create a virtual memory arena that's too small for the full complement of zones, round up the size to the minimum instead of asserting.

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 Change: 187164
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mps/code/arenacl.c

@@ -257,6 +257,10 @@ static Res ClientArenaInit(Arena *arenaReturn, ArenaClass class, ArgList args)
   AVER(base != (Addr)0);
   AVERT(ArenaGrainSize, grainSize);
 
+  if (size < grainSize * MPS_WORD_SHIFT)
+    /* Not enough room for a full complement of zones. */
+    return ResMEMORY;
+
   clArenaSize = SizeAlignUp(sizeof(ClientArenaStruct), MPS_PF_ALIGN);
   if (size < clArenaSize)
     return ResMEMORY;

mps/code/arenavm.c

@@ -507,6 +507,7 @@ static Res VMArenaInit(Arena *arenaReturn, ArenaClass class, ArgList args)
 {
   Size size = VM_ARENA_SIZE_DEFAULT; /* initial arena size */
   Align grainSize = MPS_PF_ALIGN; /* arena grain size */
+  Size pageSize = PageSize(); /* operating system page size */
   Size chunkSize; /* size actually created */
   Size vmArenaSize; /* aligned size of VMArenaStruct */
   Res res;
@@ -522,14 +523,19 @@ static Res VMArenaInit(Arena *arenaReturn, ArenaClass class, ArgList args)
   AVER(class == VMArenaClassGet());
   AVERT(ArgList, args);
 
-  if (ArgPick(&arg, args, MPS_KEY_ARENA_SIZE))
-    size = arg.val.size;
   if (ArgPick(&arg, args, MPS_KEY_ARENA_GRAIN_SIZE))
     grainSize = arg.val.size;
-  grainSize = SizeAlignUp(grainSize, PageSize());
-
-  AVER(size > 0);
+  if (grainSize < pageSize)
+    /* Make it easier to write portable programs by rounding up. */
+    grainSize = pageSize;
   AVERT(ArenaGrainSize, grainSize);
+
+  if (ArgPick(&arg, args, MPS_KEY_ARENA_SIZE))
+    size = arg.val.size;
+  if (size < grainSize * MPS_WORD_WIDTH)
+    /* There has to be enough room in the chunk for a full complement of
+       zones. Make it easier to write portable programs by rounding up. */
+    size = grainSize * MPS_WORD_WIDTH;
   
   /* Parse remaining arguments, if any, into VM parameters. We must do
      this into some stack-allocated memory for the moment, since we

mps/manual/source/topic/arena.rst

@@ -254,6 +254,14 @@ Virtual memory arenas
       necessary. The MPS is most efficient if you reserve an address
       space that is several times larger than your peak memory usage.
 
+      If you specify a value for :c:macro:`MPS_KEY_ARENA_SIZE` that's
+      too small for the virtual memory arena, then the MPS rounds it
+      up to the minimum and continues. The minimum size for the
+      virtual memory arena is :c:macro:`MPS_WORD_WIDTH` ×
+      :c:macro:`MPS_KEY_ARENA_GRAIN_SIZE` bytes. For example, on a
+      64-bit platform with a 4\ :term:`kilobyte` page size, this is
+      256\ :term:`kilobytes`.
+
       .. note::
 
           The MPS asks for more address space if it runs out, but the
@@ -262,11 +270,14 @@ Virtual memory arenas
 
     * :c:macro:`MPS_KEY_ARENA_GRAIN_SIZE` (type :c:type:`size_t`) is
       the granularity with which the arena will manage memory
-      internally. It must be a power of 2. It will be aligned up to a
-      multiple of the operating system's page size if necessary. If
-      not provided, the operating system's page size is used. Larger
-      granularity reduces overheads, but increases
-      :term:`fragmentation` and :term:`retention`.
+      internally. It must be a power of 2. If not provided, the
+      operating system's page size is used. Larger granularity reduces
+      overheads, but increases :term:`fragmentation` and
+      :term:`retention`.
+
+      If you specify a value of :c:macro:`MPS_KEY_ARENA_GRAIN_SIZE`
+      that's smaller than the operating system page size, the MPS
+      rounds it up to the page size and continues.
 
     A second optional :term:`keyword argument` may be passed, but it
     only has any effect on the Windows operating system:

mps/test/testsets/passing

@@ -109,7 +109,7 @@ function/117.c
 function/118.c
 function/119.c
 function/120.c
-% function/121.c -- job003495
+function/121.c -- job003495
 function/122.c
 function/123.c
 function/124.c