Re: [PATCH 5/6] mm: document grub_mm_init_region

[Daniel Axtens]

Daniel Kiper <dkiper@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 <dja@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