Re: [PATCH] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[Johannes Weiner]

On Tue, Nov 24, 2015 at 06:02:39PM +0100, Michal Hocko wrote:
> On Tue 24-11-15 11:26:04, Johannes Weiner wrote:
> > On Tue, Nov 24, 2015 at 10:47:09AM +0100, Michal Hocko wrote:
> > > Besides that there is no other reliable warning that we are getting
> > > _really_ short on memory unlike when the allocation failure is
> > > allowed. OOM killer report might be missing because there was no actual
> > > killing happening.
> > 
> > This is why I would like to see that warning generalized, and not just
> > for __GFP_NOFAIL. We have allocations other than explicit __GFP_NOFAIL
> > that will loop forever in the allocator,
> 
> Yes but does it make sense to warn for all of them? Wouldn't it be
> sufficient to warn about those which cannot allocate anything even
> though they are doing ALLOC_NO_WATERMARKS?

Why is it important whether they can do ALLOC_NO_WATERMARKS or not?

I'm worried about all those that can loop forever with locks held.

> > and when this deadlocks the
> > machine all we see is other tasks hanging, but not the culprit. If we
> > were to get a backtrace of some task in the allocator that is known to
> > hold locks, suddenly all the other hung tasks will make sense, and it
> > will clearly distinguish such an allocator deadlock from other issues.
> 
> Tetsuo was suggesting a more sophisticated infrastructure for tracking
> allocations [1] which take too long without making progress. I haven't
> seen his patch because I was too busy with other stuff but maybe this is
> what you would like to see?

That seems a bit excessive. I was thinking something more like this:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 05ef7fb..fbfc581 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3004,6 +3004,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	enum migrate_mode migration_mode = MIGRATE_ASYNC;
 	bool deferred_compaction = false;
 	int contended_compaction = COMPACT_CONTENDED_NONE;
+	unsigned int nr_tries = 0;
 
 	/*
 	 * In the slowpath, we sanity check order to avoid ever trying to
@@ -3033,6 +3034,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		goto nopage;
 
 retry:
+	if (++nr_retries % 100 == 0)
+		warn_alloc_failed(gfp_mask, order, "Potential GFP deadlock\n");
+
 	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
 		wake_all_kswapds(order, ac);
 
> Anyway I would like to make some progress on this patch. Do you think
> that it would be acceptable in the current form without the warning or
> you preffer a different way?

Oh, I have nothing against your patch, please go ahead with it. I just
wondered out loud when you proposed a warning about deadlocking NOFAIL
allocations but limited it to explicit __GFP_NOFAIL allocations, when
those obviously aren't the only ones that can deadlock in that way.
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