121 lines
3.3 KiB
Diff
121 lines
3.3 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Thu, 25 Nov 2021 02:22:48 +1100
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Subject: [PATCH] mm: Document grub_free()
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The grub_free() possesses a surprising number of quirks, and also
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uses single-letter variable names confusingly to iterate through
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the free list.
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Document what's going on.
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Use prev and cur to iterate over the free list.
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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(cherry picked from commit 1f8d0b01738e49767d662d6426af3570a64565f0)
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---
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grub-core/kern/mm.c | 63 ++++++++++++++++++++++++++++++++++-------------------
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1 file changed, 41 insertions(+), 22 deletions(-)
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diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
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index db7e0b2a5b..0351171cf9 100644
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--- a/grub-core/kern/mm.c
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+++ b/grub-core/kern/mm.c
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@@ -446,54 +446,73 @@ grub_free (void *ptr)
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}
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else
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{
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- grub_mm_header_t q, s;
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+ grub_mm_header_t cur, prev;
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#if 0
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- q = r->first;
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+ cur = r->first;
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do
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{
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grub_printf ("%s:%d: q=%p, q->size=0x%x, q->magic=0x%x\n",
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- GRUB_FILE, __LINE__, q, q->size, q->magic);
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- q = q->next;
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+ GRUB_FILE, __LINE__, cur, cur->size, cur->magic);
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+ cur = cur->next;
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}
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- while (q != r->first);
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+ while (cur != r->first);
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#endif
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-
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- for (s = r->first, q = s->next; q <= p || q->next >= p; s = q, q = s->next)
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+ /* Iterate over all blocks in the free ring.
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+ *
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+ * The free ring is arranged from high addresses to low
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+ * addresses, modulo wraparound.
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+ *
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+ * We are looking for a block with a higher address than p or
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+ * whose next address is lower than p.
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+ */
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+ for (prev = r->first, cur = prev->next; cur <= p || cur->next >= p;
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+ prev = cur, cur = prev->next)
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{
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- if (q->magic != GRUB_MM_FREE_MAGIC)
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- grub_fatal ("free magic is broken at %p: 0x%x", q, q->magic);
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+ if (cur->magic != GRUB_MM_FREE_MAGIC)
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+ grub_fatal ("free magic is broken at %p: 0x%x", cur, cur->magic);
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- if (q <= q->next && (q > p || q->next < p))
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+ /* Deal with wrap-around */
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+ if (cur <= cur->next && (cur > p || cur->next < p))
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break;
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}
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+ /* mark p as free and insert it between cur and cur->next */
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p->magic = GRUB_MM_FREE_MAGIC;
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- p->next = q->next;
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- q->next = p;
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+ p->next = cur->next;
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+ cur->next = p;
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+ /*
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+ * If the block we are freeing can be merged with the next
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+ * free block, do that.
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+ */
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if (p->next + p->next->size == p)
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{
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p->magic = 0;
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p->next->size += p->size;
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- q->next = p->next;
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+ cur->next = p->next;
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p = p->next;
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}
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- r->first = q;
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+ r->first = cur;
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- if (q == p + p->size)
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+ /* Likewise if can be merged with the preceeding free block */
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+ if (cur == p + p->size)
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{
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- q->magic = 0;
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- p->size += q->size;
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- if (q == s)
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- s = p;
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- s->next = p;
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- q = s;
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+ cur->magic = 0;
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+ p->size += cur->size;
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+ if (cur == prev)
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+ prev = p;
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+ prev->next = p;
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+ cur = prev;
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}
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- r->first = q;
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+ /*
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+ * Set r->first such that the just free()d block is tried first.
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+ * (An allocation is tried from *first->next, and cur->next == p.)
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+ */
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+ r->first = cur;
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}
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}
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