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SubjectRe: [patch 3/4] mempolicy: add MPOL_F_STATIC_NODES flag
On Wed, 13 Feb 2008, Lee Schermerhorn wrote:

> > > If we just want to preserve existing behavior, we can define an
> > > additional mode flag that we set in the sbinfo policy in
> > > shmem_parse_mpol() and test in mpol_new(). If we want to be able to
> > > specify existing or new behavior, we can use the same flag, but set it
> > > or not based on an additional qualifier specified via the mount option.
> > >
> >
> > Shmem areas between cpusets with disjoint mems_allowed seems like an error
> > in userspace to me and I would prefer that mpol_new() reject it outright.
>
> I don't think so, and I'd like to explain why. Perhaps this is too big
> a topic to cover in the context of this patch response. I think I'll
> start another thread. Suffice it to say here that cpusets and
> mempolicies don't constrain, in anyway, the ability of tasks in a cpuset
> to attach to shmem segments created by tasks in other, perhaps disjoint,
> cpusets. The cpusets and mempolicies DO constrain page allocations,
> however. This can result in OOM faults for tasks in cpusets whose
> mems_allowed contains no overlap with a shared policy installed by the
> creating task. This can be avoided if the task that creates the shmem
> segment faults in all of the pages and SHM_LOCKs them down. Whether
> this is sufficient for all applications is subject of the other
> discussion. In any case, I think we need to point this out in mempolicy
> and cpuset man pages.
>

I don't think we're likely to see examples of actual code written to do
this being posted to refute my comment.

I think what you said above is right and that it should be allowed.
You're advocating for allowing task A to attach to shmem segments created
by task B while task A has its own mempolicy that restricts its own
allocations but still being able to access the shmem segments of task B,
providing that task A will not fault them back in itself.

You're right, that's not a scenario that I was hoping to address to
MPOL_F_STATIC_NODES.

> > Since we're allowed to remap the node to a different node than the user
> > specified with either syscall, the current behavior is that "one node is
> > as good as another." In other words, we're trying to accomodate the mode
> > first by setting pol->v.preferred_node to some accessible node and only
> > setting that to the user-supplied node if it is available.
> >
> > If the node isn't available and the user specifically asked that it is not
> > remapped (with MPOL_F_STATIC_NODES), then I felt local allocation was best
> > compared to remapping to a node that may be unrelated to the VMA or task.
> > This preserves a sense of the current behavior that "one node is as good
> > as another," but the user specifically asked for no remap.
>
> Yeah. I went back and read the update to the mempolicy doc where you
> make it clear that this is the semantic of 'STATIC_NODES. You also
> mentioned it in the patch description, but I didn't "get it". Sorry.
>
> Still a comment in the code would, I think, help future spelunkers.
>

Ok, I'll clarify the intention in this code that we agreed was better:

case MPOL_PREFERRED:
if (!remap) {
int nid = first_node(pol->user_nodemask);

if (node_isset(nid, *newmask))
pol->v.preferred_node = nid;
else
pol->v.preferred_node = -1;
} else
pol->v.preferred_node = node_remap(pol->v.preferred_node,
*mpolmask, *newmask);
break;


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