Memory management broken by "mm: reclaim small amounts of memory when an external fragmentation event occurs"

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Hi

The patch 1c30844d2dfe272d58c8fc000960b835d13aa2ac ("mm: reclaim small 
amounts of memory when an external fragmentation event occurs") breaks 
memory management on parisc.

I have a parisc machine with 7GiB RAM, the chipset maps the physical 
memory to three zones:
	0) Start 0x0000000000000000 End 0x000000003fffffff Size   1024 MB
	1) Start 0x0000000100000000 End 0x00000001bfdfffff Size   3070 MB
	2) Start 0x0000004040000000 End 0x00000040ffffffff Size   3072 MB
(but it is not NUMA)

With the patch 1c30844d2, the kernel will incorrectly reclaim the first 
zone when it fills up, ignoring the fact that there are two completely 
free zones. Basiscally, it limits cache size to 1GiB.

For example, if I run:
# dd if=/dev/sda of=/dev/null bs=1M count=2048

- with the proper kernel, there should be "Buffers - 2GiB" when this 
command finishes. With the patch 1c30844d2, buffers will consume just 1GiB 
or slightly more, because the kernel was incorrectly reclaiming them.

Mikulas



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