[PATCH v3 1/2] mm: Adjust new region size to take management overhead into account

[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, the size passed to it doesn't take region management overhead
into account. Adding a memory area of "size" bytes may result in a heap
region of less than "size" bytes truely avaliable. Thus, the new region
may not be adequate for current allocation request, confusing
out-of-memory handling code.

This patch introduces GRUB_MM_MGMT_OVERHEAD to address the region
management overhead (e.g. metadata, padding). The value of this new
constant must be large enough to make sure grub_malloc(size) always
success after a successful call to grub_mm_init_region(addr, size +
GRUB_MM_MGMT_OVERHEAD), for any given addr and size (assuming no
interger overflow).

The size passed to grub_mm_add_region_fn() is now set to "size + align +
GRUB_MM_MGMT_OVERHEAD", thus if grub_mm_add_region_fn() succeeded,
current allocation request can always success.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
---
 grub-core/kern/mm.c | 54 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
index ae2279133..5c0e5bbbf 100644
--- a/grub-core/kern/mm.c
+++ b/grub-core/kern/mm.c
@@ -83,6 +83,43 @@
 
 
 
+/*
+ * GRUB_MM_MGMT_OVERHEAD is an upper bound of management overhead of
+ * each region, with any possible padding taken into account.
+ *
+ * The value must be large enough to make sure grub_malloc(size) always
+ * success after a successful call to
+ * grub_mm_init_region(addr, size + GRUB_MM_MGMT_OVERHEAD), for any given
+ * addr and size (assuming no interger overflow).
+ *
+ * The worst case which has maximum overhead is shown in the figure below:
+ *
+ * +-- addr
+ * v                                           |<-   size   ->|
+ * +---------+----------------+----------------+--------------+---------+
+ * | padding | grub_mm_region | grub_mm_header | usable bytes | padding |
+ * +---------+----------------+----------------+--------------+---------+
+ * |<-  a  ->|<-     b      ->|<-     c      ->|<-    d     ->|<-  e  ->|
+ *                                             ^
+ *    addr % GRUB_MM_ALIGN == 1                +--/ This will be the pointer
+ *    a == GRUB_MM_ALIGN - 1                      | returned by next
+ *                                                | grub_malloc(size),
+ *    b == sizeof (struct grub_mm_region)         | if no other suitable free
+ *    c == sizeof (struct grub_mm_header)         \ block is available.
+ *
+ *    size % GRUB_MM_ALIGN == 1
+ *    d == size
+ *    e == GRUB_MM_ALIGN - 1
+ *
+ * Therefore, the maximum overhead is:
+ *    a + b + c + e == (GRUB_MM_ALIGN - 1) + sizeof (struct grub_mm_region)
+ *                     + sizeof (struct grub_mm_header) + (GRUB_MM_ALIGN - 1)
+ */
+#define GRUB_MM_MGMT_OVERHEAD ((GRUB_MM_ALIGN - 1) \
+                              + sizeof (struct grub_mm_region) \
+                              + sizeof (struct grub_mm_header) \
+                              + (GRUB_MM_ALIGN - 1))
+
 grub_mm_region_t grub_mm_base;
 grub_mm_add_region_func_t grub_mm_add_region_fn;
 
@@ -230,6 +267,11 @@ grub_mm_init_region (void *addr, grub_size_t size)
 
   grub_dprintf ("regions", "No: considering a new region at %p of size %" 
PRIxGRUB_SIZE "\n",
                addr, size);
+  /*
+   * If you want to modify the code below, please also take a look at
+   * GRUB_MM_MGMT_OVERHEAD and make sure it is synchronized with the code.
+   */
+
   /* Allocate a region from the head.  */
   r = (grub_mm_region_t) ALIGN_UP ((grub_addr_t) addr, GRUB_MM_ALIGN);
 
@@ -410,6 +452,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
 {
   grub_mm_region_t r;
   grub_size_t n = ((size + GRUB_MM_ALIGN - 1) >> GRUB_MM_ALIGN_LOG2) + 1;
+  grub_size_t grow;
   int count = 0;
 
   if (!grub_mm_base)
@@ -424,6 +467,13 @@ grub_memalign (grub_size_t align, grub_size_t size)
   if ((size + align) > ~(grub_size_t) 0x100000)
     goto fail;
 
+  /*
+   * Pre-calculate the size of heap growth (if applicable),
+   * with region management overhead taken into account.
+   */
+  if (grub_add (size + align, GRUB_MM_MGMT_OVERHEAD, &grow))
+    goto fail;
+
   align = (align >> GRUB_MM_ALIGN_LOG2);
   if (align == 0)
     align = 1;
@@ -447,7 +497,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
       count++;
 
       if (grub_mm_add_region_fn != NULL &&
-          grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_CONSECUTIVE) == 
GRUB_ERR_NONE)
+          grub_mm_add_region_fn (grow, GRUB_MM_ADD_REGION_CONSECUTIVE) == 
GRUB_ERR_NONE)
        goto again;
 
       /* fallthrough  */
@@ -462,7 +512,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
            * Try again even if this fails, in case it was able to partially
            * satisfy the request
            */
-          grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_NONE);
+          grub_mm_add_region_fn (grow, GRUB_MM_ADD_REGION_NONE);
           goto again;
         }
 
-- 
2.30.2