Re: [PATCH] Memoryless nodes: use "node_memory_map" for cpuset mems_allowed validation

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

On Mon, 2007-07-23 at 12:09 -0700, Nishanth Aravamudan wrote: 
> On 20.07.2007 [16:49:24 -0400], Lee Schermerhorn wrote:
> > This fixes a problem I encountered testing Christoph's memoryless nodes
> > series.  Applies atop that series.  Other than this, series holds up
> > under what testing I've been able to do this week.
> > 
> > Memoryless Nodes:  use "node_memory_map" for cpusets mems_allowed validation
> > 
> > cpusets try to ensure that any node added to a cpuset's 
> > mems_allowed is on-line and contains memory.  The assumption
> > was that online nodes contained memory.  Thus, it is possible
> > to add memoryless nodes to a cpuset and then add tasks to this
> > cpuset.  This results in continuous series of oom-kill and other
> > console stack traces and apparent system hang.
> > 
> > Change cpusets to use node_states[N_MEMORY] [a.k.a.
> > node_memory_map] in place of node_online_map when vetting 
> > memories.  Return error if admin attempts to write a non-empty
> > mems_allowed node mask containing only memoryless-nodes.
> > 
> > Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>
> 
> Lee, while looking at this change, I think it ends up fixing
> cpuset_mems_allowed() to return nodemasks that only include nodes in
> node_states[N_MEMORY]. However, cpuset_current_mems_allowed is a
> lockless macro which would still be broken. I think it would need to
> becom a static inline nodes_and() in the CPUSET case and a #define
> node_states[N_MEMORY] in the non-CPUSET case?
> 
> Or perhaps we should adjust cpusets to make it so that the mems_allowed
> member only includes nodes that are set in node_states[N_MEMORY]?


I thought that's what my patch to nodelist_parse() did.  It ensures that
current->mems_allowed is correct [contains at least one node with
memory, and only nodes with memory] at the time it is installed, but
doesn't consider memory hot plug and node off-lining.  Is this
[offline/hotplug] your point?

Seems like that is an issue that exists in the unpatched code as
well--i.e., unlike cpuset_mems_allowed(), the lockless, "_current_"
version does not vet current->mems_allowed against the
nodes_online_mask.  So, all valid nodes in current->mems_allowed could
have been off-lined since the mask was installed.  Am I reading this
right?

Lee

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