Do not group pages by mobility type on low memory systems

Grouping pages by mobility can only successfully operate when there are more
MAX_ORDER_NR_PAGES areas than mobility types.  When there are insufficient
areas, fallbacks cannot be avoided.  This has noticeable performance impacts
on machines with small amounts of memory in comparison to MAX_ORDER_NR_PAGES.
For example, on IA64 with a configuration including huge pages spans 1GiB with
MAX_ORDER_NR_PAGES so would need at least 4GiB of RAM before grouping pages by
mobility would be useful.  In comparison, an x86 would need 16MB.

This patch checks the size of vm_total_pages in build_all_zonelists(). If
there are not enough areas,  mobility is effectivly disabled by considering
all allocations as the same type (UNMOVABLE).  This is achived via a
__read_mostly flag.

With this patch, performance is comparable to disabling grouping pages
by mobility at compile-time on a test machine with insufficient memory.
With this patch, it is reasonable to get rid of grouping pages by mobility
a compile-time option.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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