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mm: Document grub_mm_init_region()

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The grub_mm_init_region() does some things that seem magical, especially
around region merging. Make it a bit clearer.

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 02:22:49 +11:00 committed by Daniel Kiper
parent 1f8d0b0173
commit 246d69b7ea
1 changed files with 30 additions and 1 deletions
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31
grub-core/kern/mm.c
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@ -128,23 +128,52 @@ grub_mm_init_region (void *addr, grub_size_t size)
if (((grub_addr_t) addr + 0x1000) > ~(grub_addr_t) size)
size = ((grub_addr_t) -0x1000) - (grub_addr_t) addr;
/* Attempt to merge this region with every existing region */
for (p = &grub_mm_base, q = *p; q; p = &(q->next), q = *p)
/*
* Is the new region immediately below an existing region? That
* is, is the address of the memory we're adding now (addr) + size
* of the memory we're adding (size) + the bytes we couldn't use
* at the start of the region we're considering (q->pre_size)
* equal to the address of q? In other words, does the memory
* looks like this?
*
* addr q
* |----size-----|-q->pre_size-|<q region>|
*/
if ((grub_uint8_t *) addr + size + q->pre_size == (grub_uint8_t *) q)
{
/*
* Yes, we can merge the memory starting at addr into the
* existing region from below. Align up addr to GRUB_MM_ALIGN
* so that our new region has proper alignment.
*/
r = (grub_mm_region_t) ALIGN_UP ((grub_addr_t) addr, GRUB_MM_ALIGN);
/* Copy the region data across */
*r = *q;
/* Consider all the new size as pre-size */
r->pre_size += size;
/*
* If we have enough pre-size to create a block, create a
* block with it. Mark it as allocated and pass it to
* grub_free (), which will sort out getting it into the free
* list.
*/
if (r->pre_size >> GRUB_MM_ALIGN_LOG2)
{
h = (grub_mm_header_t) (r + 1);
/* block size is pre-size converted to cells */
h->size = (r->pre_size >> GRUB_MM_ALIGN_LOG2);
h->magic = GRUB_MM_ALLOC_MAGIC;
/* region size grows by block size converted back to bytes */
r->size += h->size << GRUB_MM_ALIGN_LOG2;
/* adjust pre_size to be accurate */
r->pre_size &= (GRUB_MM_ALIGN - 1);
*p = r;
grub_free (h + 1);
}
/* Replace the old region with the new region */
*p = r;
return;
}