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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 02:22:49 +1100
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Subject: [PATCH] mm: Document grub_mm_init_region()
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The grub_mm_init_region() does some things that seem magical, especially
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around region merging. Make it a bit clearer.
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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 246d69b7ea619fc1e77dcc5960e37aea45a9808c)
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---
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grub-core/kern/mm.c | 31 ++++++++++++++++++++++++++++++-
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1 file changed, 30 insertions(+), 1 deletion(-)
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diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
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index 0351171cf9..1cbf98c7ab 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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@@ -128,23 +128,52 @@ grub_mm_init_region (void *addr, grub_size_t size)
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if (((grub_addr_t) addr + 0x1000) > ~(grub_addr_t) size)
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size = ((grub_addr_t) -0x1000) - (grub_addr_t) addr;
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+ /* Attempt to merge this region with every existing region */
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for (p = &grub_mm_base, q = *p; q; p = &(q->next), q = *p)
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+ /*
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+ * Is the new region immediately below an existing region? That
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+ * is, is the address of the memory we're adding now (addr) + size
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+ * of the memory we're adding (size) + the bytes we couldn't use
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+ * at the start of the region we're considering (q->pre_size)
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+ * equal to the address of q? In other words, does the memory
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+ * looks like this?
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+ *
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+ * addr q
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+ * |----size-----|-q->pre_size-|<q region>|
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+ */
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if ((grub_uint8_t *) addr + size + q->pre_size == (grub_uint8_t *) q)
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{
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+ /*
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+ * Yes, we can merge the memory starting at addr into the
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+ * existing region from below. Align up addr to GRUB_MM_ALIGN
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+ * so that our new region has proper alignment.
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+ */
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r = (grub_mm_region_t) ALIGN_UP ((grub_addr_t) addr, GRUB_MM_ALIGN);
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+ /* Copy the region data across */
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*r = *q;
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+ /* Consider all the new size as pre-size */
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r->pre_size += size;
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-
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+
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+ /*
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+ * If we have enough pre-size to create a block, create a
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+ * block with it. Mark it as allocated and pass it to
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+ * grub_free (), which will sort out getting it into the free
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+ * list.
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+ */
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if (r->pre_size >> GRUB_MM_ALIGN_LOG2)
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{
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h = (grub_mm_header_t) (r + 1);
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+ /* block size is pre-size converted to cells */
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h->size = (r->pre_size >> GRUB_MM_ALIGN_LOG2);
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h->magic = GRUB_MM_ALLOC_MAGIC;
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+ /* region size grows by block size converted back to bytes */
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r->size += h->size << GRUB_MM_ALIGN_LOG2;
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+ /* adjust pre_size to be accurate */
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r->pre_size &= (GRUB_MM_ALIGN - 1);
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*p = r;
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grub_free (h + 1);
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}
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+ /* Replace the old region with the new region */
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*p = r;
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return;
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}
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