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Re: [PATCH] Huge changes in mm.c

From: Vincent Guffens
Subject: Re: [PATCH] Huge changes in mm.c
Date: Mon, 11 Jul 2005 15:09:56 +0200
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Vincent Pelletier wrote:
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In my attempt to port grub 2 to usparc, I had big problems with all the
memory allocation things, so I decided to rewrite the functions after
reading them (I wasn't able to understand at all 20% of the code).

Some comments :
It *seems* that before the alloc'ed chunks weren't linked any more with
other chunks. Now all the chunks of a region are part of the same ring.
Chunks were allocated from the end of the memory to the beginning, I
kept this behaviour.

From what I understood by reading the sources, the allocated chunks were not linked to other free chunks as you can easily recover the header of such an allocated memory chunk by using the pointer returned by the malloc function. This is simply p-1. It is not possible to free an allocated region without that pointer anyway as it is not possible to know if someone is still using that memory region. It must be released by the one who called the malloc.

It *seems* that the chunk headers were "forgotten" in the malloc
process, which led to my problems on usparc : a chunk was overwriting
the header of the previous one. It's now fixed.

Maybe it is specific of that architecture. The function grub_malloc calls grub_memalign (0, size). Then the size is converted in blocks with

grub_size_t n = ((size + GRUB_MM_ALIGN - 1) >> GRUB_MM_ALIGN_LOG2) + 1;

which adds the extra block for the header. Which headers were overwritten exactly, I spent quite some time on that code and I couldn't see any problem except for the "grub free magic is broken" problem but off course it was not with that architecture in mind.

                                Vincent Guffens
                                PhD Student UCL/CESAME
                                tel:   +32 10 47 80 30
Value your freedom, or you will lose it, teaches history.
"Don't bother us with politics," respond those who don't want to learn.
                -- Richard M. Stallman

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