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mm: Document GRUB internal memory management structures

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I spent more than a trivial quantity of time figuring out pre_size and
whether a memory region's size contains the header cell or not.

Document the meanings of all the properties. Hopefully now no-one else
has to figure it out!

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Daniel Axtens 2021-11-25 02:22:45 +11:00 committed by Daniel Kiper
parent b0f06a81c6
commit a6c5c52ccf
1 changed files with 28 additions and 0 deletions
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28
include/grub/mm_private.h
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@ -21,15 +21,27 @@
#include <grub/mm.h>
/* For context, see kern/mm.c */
/* Magic words. */
#define GRUB_MM_FREE_MAGIC 0x2d3c2808
#define GRUB_MM_ALLOC_MAGIC 0x6db08fa4
/* A header describing a block of memory - either allocated or free */
typedef struct grub_mm_header
{
/*
* The 'next' free block in this region's circular free list.
* Only meaningful if the block is free.
*/
struct grub_mm_header *next;
/* The block size, not in bytes but the number of cells of
* GRUB_MM_ALIGN bytes. Includes the header cell.
*/
grub_size_t size;
/* either free or alloc magic, depending on the block type. */
grub_size_t magic;
/* pad to cell size: see the top of kern/mm.c. */
#if GRUB_CPU_SIZEOF_VOID_P == 4
char padding[4];
#elif GRUB_CPU_SIZEOF_VOID_P == 8
@ -48,11 +60,27 @@ typedef struct grub_mm_header
#define GRUB_MM_ALIGN (1 << GRUB_MM_ALIGN_LOG2)
/* A region from which we can make allocations. */
typedef struct grub_mm_region
{
/* The first free block in this region. */
struct grub_mm_header *first;
/*
* The next region in the linked list of regions. Regions are initially
* sorted in order of increasing size, but can grow, in which case the
* ordering may not be preserved.
*/
struct grub_mm_region *next;
/*
* A grub_mm_region will always be aligned to cell size. The pre-size is
* the number of bytes we were given but had to skip in order to get that
* alignment.
*/
grub_size_t pre_size;
/* How many bytes are in this region? (free and allocated) */
grub_size_t size;
}
*grub_mm_region_t;