[PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation

[Christoph Lameter]

On Thu, 28 Jan 2021, Michal Hocko wrote:

> > > If you kill the allocating process then yes, it would work, but your
> > > process might be the very last to be selected.
> >
> > OOMs are different if you have a "constrained allocation". In that case it
> > is the fault of the process who wanted memory with certain conditions.
> > That memory is not available. General memory is available though. In that
> > case the allocating process is killed.
>
> I do not see this implementation would do anything like that. Neither
> anything like that implemented in the oom killer. Constrained
> allocations (cpusets/memcg/mempolicy) only do restrict their selection
> to processes which belong to the same domain. So I am not really sure
> what you are referring to. The is only a global knob to _always_ kill
> the allocating process on OOM.

Constrained allocations refer to allocations where the NUMA nodes are
restricted or something else does not allow the use of arbitrary memory.
The OOM killer changes its behavior. In the past we fell back to killing
the calling process.

See constrained_alloc() in mm/oom_kill.c

static const char * const oom_constraint_text[] = {
        [CONSTRAINT_NONE] = "CONSTRAINT_NONE",
        [CONSTRAINT_CPUSET] = "CONSTRAINT_CPUSET",
        [CONSTRAINT_MEMORY_POLICY] = "CONSTRAINT_MEMORY_POLICY",
        [CONSTRAINT_MEMCG] = "CONSTRAINT_MEMCG",
};

/*
 * Determine the type of allocation constraint.
 */
static enum oom_constraint constrained_alloc(struct oom_control *oc)
{