[PATCH 2/2] mm: Preallocate some space when adding new regions

[Zhang Boyang]

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, which may be
too slow if those requests are small (e.g. about 4KB).

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

The value of GRUB_MM_HEAP_GROW 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>
---
 grub-core/kern/mm.c       | 3 +++
 include/grub/mm_private.h | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
index 973cb6b15..83c618c5b 100644
--- a/grub-core/kern/mm.c
+++ b/grub-core/kern/mm.c
@@ -452,6 +452,9 @@ grub_memalign (grub_size_t align, grub_size_t size)
       if (grub_add (bound, GRUB_MM_MAX_COST, &grow))
        goto fail;
 
+      /* Calculate optimal size of heap growth. */
+      grow = grub_max (grow, GRUB_MM_HEAP_GROW);
+
       /* Request additional pages, contiguous */
       count++;
 
diff --git a/include/grub/mm_private.h b/include/grub/mm_private.h
index f212110e4..268fbf094 100644
--- a/include/grub/mm_private.h
+++ b/include/grub/mm_private.h
@@ -104,6 +104,9 @@ typedef struct grub_mm_region
  */
 #define GRUB_MM_MAX_COST 0x1000
 
+/* Minimal heap growth granularity when existing heap space is exhausted. */
+#define GRUB_MM_HEAP_GROW 0x100000
+
 #ifndef GRUB_MACHINE_EMU
 extern grub_mm_region_t EXPORT_VAR (grub_mm_base);
 #endif
-- 
2.30.2