From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Daniel Axtens Date: Thu, 25 Nov 2021 02:22:49 +1100 Subject: [PATCH] mm: Document grub_mm_init_region() 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 Reviewed-by: Daniel Kiper (cherry picked from commit 246d69b7ea619fc1e77dcc5960e37aea45a9808c) --- grub-core/kern/mm.c | 31 ++++++++++++++++++++++++++++++- 1 file changed, 30 insertions(+), 1 deletion(-) diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c index 0351171cf9..1cbf98c7ab 100644 --- a/grub-core/kern/mm.c +++ b/grub-core/kern/mm.c @@ -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-|| + */ 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; }