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

[Tetsuo Handa]

Tetsuo Handa wrote:
> 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.

According to kmallocwd, allocating tasks are very slowly able to call
out_of_memory() ( http://I-love.SAKURA.ne.jp/tmp/serial-20160313.txt.xz ).
It seems that the oom detection rework patches are not really related.

> 
> 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.
> 

The oom detection rework patches did not add a new location of livelock.
They just did not address the problem that I/O cannot make progress.