Am 01.12.2011 16:39, schrieb Uwe Kleine-K?nig:

Hello,

On Thu, Dec 01, 2011 at 11:57:18AM +0100, Uwe Kleine-K?nig wrote:

quoted

On Tue, Nov 29, 2011 at 10:39:10PM -0200, Fabio Estevam wrote:

quoted

2011/11/29 Uwe Kleine-K?nig [off-list ref]:

quoted

Hello,

I'm currently working on a new arch port and my current machine has RAM
at 0x10000000 and 0x80000000. So there is a big hole between the two
banks. When selecting the sparse memory model it works, but when
selecting flat the machine runs into a BUG in mark_bootmem() called by
free_unused_memmap() to free the space between the two banks.

My understanding is that you have to select ARCH_HAS_HOLES_MEMORYMODEL.

I think that is not necessary.
 

quoted

quoted

Is that expected (meaning I cannot use the flat model)? I currently
don't have another machine handy that has >1 memory back to test that.

In case you have access to a MX35PDK you can try on this board as it does have
the memory hole.

No I havn't, but I just used a 128MB machine and changed that in the
.fixup callback to 64MB + 32MB with a 32MB hole in between and it works
fine without ARCH_HAS_HOLES_MEMORYMODEL.

I debugged the problem a bit further and one symptom is that

	struct page *mem_map

is NULL for me. That looks wrong. I guess this is just broken for nommu.
I will dig into that later today.

The problem is that the memory for mem_map is allocated using:

	map = alloc_bootmem_node_nopanic(pgdat, size);

without any error checking. The _nopanic was introduced by commit

	8f389a99 (mm: use alloc_bootmem_node_nopanic() on really needed path)

I don't understand the commit's log and don't really see why it should
be allowed to not panic if the allocation failes here but use a NULL
pointer instead.
I put the people involved in 8f389a99 on Cc, maybe someone can comment?

Apart from that it seems I cannot use flatmem as is on my machine. It
has only 128kiB at 0x10000000 + 1MiB at 0x80000000 and needs 14MiB to hold the
table of "struct page"s. :-(

Best regards
Uwe

The commit was made after an bug report from me. I have an old x86
tablet pc with only 8Mb Ram. This machine fails early on bootup without
this commit. I found an archived message of the bug report here:
http://comments.gmane.org/gmane.linux.kernel/1135909

Regards,
Stefan