Re: [RFC][PATCH 4/5] hugetlb: fix cpuset-constrained pool resizing

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

On Mon, 2007-08-06 at 11:04 -0700, Christoph Lameter wrote:
> On Mon, 6 Aug 2007, Nishanth Aravamudan wrote:
> 
> > hugetlb: fix cpuset-constrained pool resizing
> > 
> > With the previous 3 patches in this series applied, if a process is in a
> > constrained cpuset, and tries to grow the hugetlb pool, hugepages may be
> > allocated on nodes outside of the process' cpuset. More concretely,
> > growing the pool via
> > 
> > echo some_value > /proc/sys/vm/nr_hugepages
> > 
> > interleaves across all nodes with memory such that hugepage allocations
> > occur on nodes outside the cpuset. Similarly, this process is able to
> > change the values in values in
> > /sys/devices/system/node/nodeX/nr_hugepages, even when X is not in the
> > cpuset. This directly violates the isolation that cpusets is supposed to
> > guarantee.
> 
> No it does not. Cpusets do not affect the administrative rights of users.

I agree.  nr_hugepages allocates fresh pages for the system wide pool.
I don't think this should not be constrained by cpusets.  I supposed
that if there is a need for this feature, we could document the behavior
and warn admins to only modify nr_hugepages from a program/shell in the
top level cpuset to achieve the current system-wide behavior.

>  
> > For pool growth: fix the sysctl case by only interleaving across the
> > nodes in current's cpuset; fix the sysfs attribute case by verifying the
> > requested node is in current's cpuset. For pool shrinking: both cases
> > are mostly already covered by the cpuset_zone_allowed_softwall() check
> > in dequeue_huge_page_node(), but make sure that we only iterate over the
> > cpusets's nodes in try_to_free_low().
> 
> In that case the number of huge pages is a cpuset attribute. Create 
> nr_hugepages under /dev/cpuset/ ...? The sysctl is global and should not 
> be cpuset relative.
>  
> Otherwise the /proc/sys/vm/nr_hugepages and systecl becomes dependend on 
> the cpuset context. Which will be a bit strange.

I'd like to see it stay a system-wide attribute to preserve current
behavior--with the fixes for memoryless nodes, of course.

I'll queue these up for testing atop Christoph's v5 memoryless nodes
patches.


Lee

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