Re: [PATCH 0/3] OOM detection rework v4

[Tetsuo Handa]

Michal Hocko wrote:
> OK, that would suggest that the oom rework patches are not really
> related. They just moved from the livelock to a sleep which is good in
> general IMHO. We even know that it is most probably the IO that is the
> problem because we know that more than half of the reclaimable memory is
> either dirty or under writeback. That is where you should be looking.
> Why the IO is not making progress or such a slow progress.
> 

A footnote. Regarding this reproducer, the problem was "anybody can declare
OOM and call out_of_memory(). But out_of_memory() does nothing because there
is a thread which has TIF_MEMDIE." before the OOM detection rework patches,
and the problem is "nobody can declare OOM and call out_of_memory(). Although
out_of_memory() will do nothing because there is a thread which has
TIF_MEMDIE." after the OOM detection rework patches.

Dave Chinner wrote at http://lkml.kernel.org/r/20160211225929.GU14668@dastard :
> > Although there are memory allocating tasks passing gfp flags with
> > __GFP_KSWAPD_RECLAIM, kswapd is unable to make forward progress because
> > it is blocked at down() called from memory reclaim path. And since it is
> > legal to block kswapd from memory reclaim path (am I correct?), I think
> > we must not assume that current_is_kswapd() check will break the infinite
> > loop condition.
> 
> Right, the threads that are blocked in writeback waiting on memory
> reclaim will be using GFP_NOFS to prevent recursion deadlocks, but
> that does not avoid the problem that kswapd can then get stuck
> on those locks, too. Hence there is no guarantee that kswapd can
> make reclaim progress if it does dirty page writeback...

Unless we address the issue Dave commented, the OOM detection rework patches
add a new location of livelock (which is demonstrated by this reproducer) in
the memory allocator. It is an unfortunate change that we add a new location
of livelock when we are trying to solve thrashing problem.