Re: How to handle TIF_MEMDIE stalls?

Michal Hocko
2015-02-20T09:10:05Z

On Thu 19-02-15 22:29:37, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Thu 19-02-15 06:01:24, Johannes Weiner wrote:
> > [...]
> > > Preferrably, we'd get rid of all nofail allocations and replace them
> > > with preallocated reserves. But this is not going to happen anytime
> > > soon, so what other option do we have than resolving this on the OOM
> > > killer side?
> >
> > As I've mentioned in other email, we might give GFP_NOFAIL allocator
> > access to memory reserves (by giving it __GFP_HIGH). This is still not a
> > 100% solution because reserves could get depleted but this risk is there
> > even with multiple oom victims. I would still argue that this would be a
> > better approach because selecting more victims might hit pathological
> > case more easily (other victims might be blocked on the very same lock
> > e.g.).
> >
> Does "multiple OOM victims" mean "select next if first does not die"?
> Then, I think my timeout patch http://marc.info/?l=linux-mm&m=142002495532320&w=2
> does not deplete memory reserves. ;-)

It doesn't because
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2603,9 +2603,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
alloc_flags |= ALLOC_NO_WATERMARKS;
else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
alloc_flags |= ALLOC_NO_WATERMARKS;
- else if (!in_interrupt() &&
- ((current->flags & PF_MEMALLOC) ||
- unlikely(test_thread_flag(TIF_MEMDIE))))
+ else if (!in_interrupt() && (current->flags & PF_MEMALLOC))
alloc_flags |= ALLOC_NO_WATERMARKS;

you disabled the TIF_MEMDIE heuristic and use it only for OOM exclusion
and break out from the allocator. Exiting task might need a memory to do
so and you make all those allocations fail basically. How do you know
this is not going to blow up?

> If we change to permit invocation of the OOM killer for GFP_NOFS / GFP_NOIO,
> those who do not want to fail (e.g. journal transaction) will start passing
> __GFP_NOFAIL?
>
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--
Michal Hocko
SUSE Labs