Re: doc on memory management

[Vincent Pelletier]

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Vincent Guffens wrote:
> I have written some doc about mm in grub2.

Nice work, explained with nice and clear schematics.
Now I understand that piece of code :).
I had trouble understanding :
 the macro you explain in "Allocation memory"
 the reason why the first free chunk of the ring is sometime moved
 the reason why alloc'ed chunks were removed from the list

I wonder why the region header contains, to me, a useless field : addr.
It is only set in grub_mm_init_region, invariably to the first header
which is invariably at the same offset from the region beginning,
defined by GRUB_MM_ALIGN. I don't think loosing sizeof(int) bytes is
critical, though, as it would be padded anyway. It only made my
comprehension of what's going on a little harder :).

I see a light improvement in grub_mm_init_region :
  /* If this region is too small, ignore it.  */
  if (size < GRUB_MM_ALIGN * 2)
    return;
If the region is 2*GRUB_MM_ALIGN, we can put a grub_mm_header_t plus a
grub_mm_region_t, and it would be full. Maybe adding 1 byte (or more)
might make more sense (but has it any practical impact...).

My realloc implementation handled the case where the realloc'ed block
had enough space after to expand. I might send a patch for this someday
if anyone finds it interesting to add.

I also implemented a memory defrag that merges successive free chunks in
one bigger. Although it relied on the fact both alloc'ed and free chucks
are known I think it can be adapted (if h->next != h+h->size*16, don't
merge h and h->next) and even a bit faster (as we only see free chunks).

Vincent Pelletier
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