mm: Preallocate some space when adding new regions

When grub_memalign() encounters out-of-memory, it will try
grub_mm_add_region_fn() to request more memory from system firmware.
However, it doesn't preallocate memory space for future allocation
requests. In extreme cases, it requires one call to
grub_mm_add_region_fn() for each memory allocation request. This can
be very slow.

This patch introduces GRUB_MM_HEAP_GROW_EXTRA, the minimal heap growth
granularity. The new region size is now set to the bigger one of its
original value and GRUB_MM_HEAP_GROW_EXTRA. Thus, it will result in some
memory space preallocated if current allocations request is small.

The value of GRUB_MM_HEAP_GROW_EXTRA is set to 1MB. If this value is
smaller, the cost of small memory allocations will be higher. If this
value is larger, more memory will be wasted and it might cause
out-of-memory on machines with small amount of RAM.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
(cherry picked from commit 21869ba)

Author: zhangboyang

grub-core/kern/mm.c

@@ -123,6 +123,9 @@
 /* The size passed to grub_mm_add_region_fn() is aligned up by this value. */
 #define GRUB_MM_HEAP_GROW_ALIGN	4096
 
+/* Minimal heap growth granularity when existing heap space is exhausted. */
+#define GRUB_MM_HEAP_GROW_EXTRA	0x100000
+
 grub_mm_region_t grub_mm_base;
 grub_mm_add_region_func_t grub_mm_add_region_fn;
 
@@ -471,6 +474,9 @@ grub_memalign (grub_size_t align, grub_size_t size)
   if (grub_add (size + align, GRUB_MM_MGMT_OVERHEAD, &grow))
     goto fail;
 
+  /* Preallocate some extra space if heap growth is small. */
+  grow = grub_max (grow, GRUB_MM_HEAP_GROW_EXTRA);
+
   /* Align up heap growth to make it friendly to CPU/MMU. */
   if (grow > ~(grub_size_t) (GRUB_MM_HEAP_GROW_ALIGN - 1))
     goto fail;