show_mem() for ia64 discontig takes a really long time on large systems.

[Robin Holt <holt@sgi.com>]

Recently, we ran a large system out of memory and the oom_kill() appeared
to have frozen up.  When we looked at the backtraces, we noticed the cpu
was making progress, but apparently not fast progress.  As a simple test,
I did a 'echo m >/proc/sysrq-trigger' and that had not completed in more
than a half-hour.

The system was a fully populated 512 node SGI machine.  The way that
memory is physically layed out results in a single pgdat which covers
the node with two holes in it.  This is new hardware with larger gaps
between the chunks of memory that earlier version had.  As show_mem()
is traversing the entire systems memory to print out stats on remaining
memory, it takes faults while trying to look at holes in the array of
struct pages.

At this point, I am looking for any sort of direction on what would be
a reasonable fix.  Should show_mem() be made to skip to a page aligned
point in the array when the fault fails?  Should we add the information
about start and end of hole to the pgdat()?  Should we have one pgdat
per chunk?  Are there other better ideas out there?  Any direction would
be greatly appreciated.

Thanks,
Robin Holt