On Thu, Nov 25, 2021 at 02:22:49AM +1100, Daniel Axtens wrote:
> grub_mm_init_region does some things that seem magical, especially
> around region merging. Make it a bit clearer.
>
> Signed-off-by: Daniel Axtens <d...@axtens.net>
> ---
> grub-core/kern/mm.c | 30 +++++++++++++++++++++++++++++-
> 1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
> index bec960c18e2b..ef2025f929de 100644
> --- a/grub-core/kern/mm.c
> +++ b/grub-core/kern/mm.c
> @@ -128,23 +128,51 @@ 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. r is our new region, it's address is the
> + * first GRUB_MM_ALIGNed address above addr.
I think s/above/below/...
> + */
> 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;
> }
Otherwise LGTM...
If you are OK with the change above then I can make it for you before
applying the patch.
Daniel
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