Daniel Kiper <dki...@net-space.pl> writes:
> 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);
We do r = ALIGN_UP(addr, GRUB_MM_ALIGN), so 'above' is actually correct.
But that probably indicates that I'm not explaining it well.
How does this sound:
/*
* 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.
*/
If you want you can change the comment and apply it, otherwise I will
submit a v2.
Kind regards,
Daniel
>> + /* 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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