Re: [PATCH] __alloc_pages - NUMA and lower zone protection

[Nick Piggin <piggin@cyberone.com.au>]

Martin Hicks wrote:

>Hi,
>
>There is a problem with the current __alloc pages on a machine with many
>nodes.  As we go down the zones[] list, we may move onto other nodes.
>Each time we go to the next zone we protect these zones by doing
>"min += local_low".
>
>This is quite appropriate on a machine with one node, but wrong on
>machines with other nodes.  To illustrate, here is an example.  On a
>256 node Altix machine, a request on node 0 for 2MB requires just over
>600MB of free memory on the 256th node in order to fullfil the "min"
>requirements if all other nodes are low on memory.  This could leave
>73GB of memory unallocated across all nodes.
>
>This patch keeps the same semantics for lower_zone_protection, but only
>provides protection for higher priority zones in the same node.
>
>The patch seems to do the right thing on my non-NUMA zx1 ia64 machine
>(which has ZONE_DMA and ZONE_NORMAL) as well as the multi-node Altix.
>
>

Could you add a comment or two, please?