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mm: Document grub_free()

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The grub_free() possesses a surprising number of quirks, and also
uses single-letter variable names confusingly to iterate through
the free list.

Document what's going on.

Use prev and cur to iterate over the free list.

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 02:22:48 +11:00 committed by Daniel Kiper
parent a847895a8d
commit 1f8d0b0173
1 changed files with 41 additions and 22 deletions
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63
grub-core/kern/mm.c
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@ -446,54 +446,73 @@ grub_free (void *ptr)
}
else
{
grub_mm_header_t q, s;
grub_mm_header_t cur, prev;
#if 0
q = r->first;
cur = r->first;
do
{
grub_printf ("%s:%d: q=%p, q->size=0x%x, q->magic=0x%x\n",
GRUB_FILE, __LINE__, q, q->size, q->magic);
q = q->next;
GRUB_FILE, __LINE__, cur, cur->size, cur->magic);
cur = cur->next;
}
while (q != r->first);
while (cur != r->first);
#endif
for (s = r->first, q = s->next; q <= p || q->next >= p; s = q, q = s->next)
/* Iterate over all blocks in the free ring.
*
* The free ring is arranged from high addresses to low
* addresses, modulo wraparound.
*
* We are looking for a block with a higher address than p or
* whose next address is lower than p.
*/
for (prev = r->first, cur = prev->next; cur <= p || cur->next >= p;
prev = cur, cur = prev->next)
{
if (q->magic != GRUB_MM_FREE_MAGIC)
grub_fatal ("free magic is broken at %p: 0x%x", q, q->magic);
if (cur->magic != GRUB_MM_FREE_MAGIC)
grub_fatal ("free magic is broken at %p: 0x%x", cur, cur->magic);
if (q <= q->next && (q > p || q->next < p))
/* Deal with wrap-around */
if (cur <= cur->next && (cur > p || cur->next < p))
break;
}
/* mark p as free and insert it between cur and cur->next */
p->magic = GRUB_MM_FREE_MAGIC;
p->next = q->next;
q->next = p;
p->next = cur->next;
cur->next = p;
/*
* If the block we are freeing can be merged with the next
* free block, do that.
*/
if (p->next + p->next->size == p)
{
p->magic = 0;
p->next->size += p->size;
q->next = p->next;
cur->next = p->next;
p = p->next;
}
r->first = q;
r->first = cur;
if (q == p + p->size)
/* Likewise if can be merged with the preceeding free block */
if (cur == p + p->size)
{
q->magic = 0;
p->size += q->size;
if (q == s)
s = p;
s->next = p;
q = s;
cur->magic = 0;
p->size += cur->size;
if (cur == prev)
prev = p;
prev->next = p;
cur = prev;
}
r->first = q;
/*
* Set r->first such that the just free()d block is tried first.
* (An allocation is tried from *first->next, and cur->next == p.)
*/
r->first = cur;
}
}