On Wed 25-03-15 02:17:10, Johannes Weiner wrote:

The zonelist locking and the oom_sem are two overlapping locks that
are used to serialize global OOM killing against different things.

The historical zonelist locking serializes OOM kills from allocations
with overlapping zonelists against each other to prevent killing more
tasks than necessary in the same memory domain.  Only when neither
tasklists nor zonelists from two concurrent OOM kills overlap (tasks
in separate memcgs bound to separate nodes) are OOM kills allowed to
execute in parallel.

The younger oom_sem is a read-write lock to serialize OOM killing
against the PM code trying to disable the OOM killer altogether.

However, the OOM killer is a fairly cold error path, there is really
no reason to optimize for highly performant and concurrent OOM kills.
And the oom_sem is just flat-out redundant.

Replace both locking schemes with a single global mutex serializing
OOM kills regardless of context.

OK, this is much simpler.

You have missed drivers/tty/sysrq.c which should take the lock as well.
ZONE_OOM_LOCKED can be removed as well. __out_of_memory in the kerneldoc
should be renamed.

[...]

OK, so we are back to what David has asked previously. We do not need
the lock for memcg and oom_killer_disabled because we know that no tasks
(except for potential oom victim) are lurking around at the time
oom_killer_disable() is called. So I guess we want to stick a comment
into mem_cgroup_oom_synchronize before we check for oom_killer_disabled.

After those are fixed, feel free to add
Acked-by: Michal Hocko <redacted>

-- 
Michal Hocko
SUSE Labs

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