Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks

[Michal Hocko <mhocko@kernel.org>]

On Wed 21-10-15 09:22:40, Christoph Lameter wrote:
> On Wed, 21 Oct 2015, Tetsuo Handa wrote:
> 
> > However, if a workqueue which is processed before vmstat_update
> > workqueue is processed got stuck inside memory allocation request,
> > values in vm_stat_diff[] cannot be merged into vm_stat[]. As a result,
> > zone_reclaimable() continues using outdated vm_stat[] values and the
> > task which is doing direct reclaim path thinks that there are reclaimable
> > pages and therefore continues looping. The consequence is a silent
> > livelock (hang up without any kernel messages) because the OOM killer
> > will not be invoked.
> 
> The diffs will be merged if they reach a certain threshold regardless. You
> can decrease that threshhold. See calculate_pressure_threshhold().

The thing is that they will not reach the threshold. The LRUs in this
particular case are empty so there is nothing scanned so
NR_PAGES_SCANNED doesn't increase.

> Why is the merging not occurring if a process gets stuck? Workrequests are
> not blocked by a process being stuck doing memory allocation or reclaim.

Because all the WQ workers are stuck somewhere, maybe in the memory
allocation which cannot make any progress and the vmstat update work is
queued behind them.

At least this is my current understanding.
-- 
Michal Hocko
SUSE Labs

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