Re: mmotm 2015-01-22-15-04: qemu failure due to 'mm: memcontrol: remove unnecessary soft limit tree node test'

[Michal Hocko]

On Fri 23-01-15 09:17:44, Christoph Lameter wrote:
> On Fri, 23 Jan 2015, Johannes Weiner wrote:
> 
> > Is the assumption of this patch wrong?  Does the specified node have
> > to be online for the fallback to work?

Admittedly, I was checking only SLAB allocator when reviewing and
assuming SLUB would behave in the same way :/
But maybe I have misinterpreted the slab code as well and
get_node(struct kmem_cache *, int node) returns non-NULL for !online
nodes.

> Nodes that are offline have no control structures allocated and thus
> allocations will likely segfault when the address of the controls
> structure for the node is accessed.
> 
> If we wanted to prevent that then every allocation would have to add a
> check to see if the nodes are online which would impact performance.

I have briefly checked the code and it seems that many users are aware
of this and use the same construct Johannes used in the end or they use
cpu_to_node. But then there are other users doing:
net/openvswitch/flow_table.c:
        /* Initialize the default stat node. */
        stats = kmem_cache_alloc_node(flow_stats_cache,
                                      GFP_KERNEL | __GFP_ZERO, 0);

and this can blow up if Node0 is not online. I haven't checked other
callers but are we sure they all are aware of !online nodes? E.g.
dev_to_node() will return a node which is assigned to a device. I do not
see where exactly this is set to anything else than -1 (I got quickly
lost in set_dev_node callers). E.g. PCI bus sets its affinity from
bus->sysdata which seems to be initialized in pci_acpi_scan_root and
that is checking for an online node. Is it possible that some devices
will get the node from BIOS or by other means?

That being said I have no problem with checking node_online in the memcg
code which was reported to blow up here. I am just thinking whether it
is safe to simply blow up like that.
-- 
Michal Hocko
SUSE Labs
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