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

[Daniel Kiper]

On Mon, Dec 13, 2021 at 11:20:22PM +1100, Daniel Axtens wrote:
> 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.

Now it is better. I will update the patch and apply it together with the
other patches which got my RB.

Thanks,

Daniel