On Tue, Aug 02, 2011 at 05:38:31PM +0530, Kautuk Consul wrote:

Hi,

In the case where the total kernel memory is not aligned to the
SECTION_SIZE_BITS I see a kernel crash.

When I copy a huge file, then the kernel crashes at the following callstack:

The callstack should not be 80-column formatted as this is completely
manged and unreadable without manual editting. Also, why did you not
include the full error message? With it, I'd have a better idea of
which bug check you hit.

Backtrace:
<SNIP>

The reason for this is that the CONFIG_HOLES_IN_ZONE configuration
option is not automatically enabled when SPARSEMEM or
ARCH_HAS_HOLES_MEMORYMODEL are enabled. Due to this, the
pfn_valid_within() macro always returns 1 due to which the BUG_ON is
encountered.
This patch enables the CONFIG_HOLES_IN_ZONE config option if either
ARCH_HAS_HOLES_MEMORYMODEL or SPARSEMEM is enabled.

Although I tested this on an older kernel, i.e., 2.6.35.13, I see that
this option has not been enabled as yet in linux-3.0 and this appears
to be a
logically correct change anyways with respect to pfn_valid_within()
functionality.

There is a performance cost associated with HOLES_IN_ZONE which may be
offset by memory savings but not necessarily.

If the BUG_ON you are hitting is this one
BUG_ON(page_zone(start_page) != page_zone(end_page)) then I'd be
wondering why the check in move_freepages_block() was insufficient.

If it's because holes are punched in the memmap then the option does
need to be set.

-- 
Mel Gorman
SUSE Labs

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