Re: [PATCH 2/2] mm: warn about ALLOC_NO_WATERMARKS request failures

[David Rientjes]

On Wed, 25 Nov 2015, Michal Hocko wrote:

> > > @@ -2642,6 +2644,13 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> > >  	if (zonelist_rescan)
> > >  		goto zonelist_scan;
> > >  
> > > +	/* WARN only once unless min_free_kbytes is updated */
> > > +	if (warn_alloc_no_wmarks && (alloc_flags & ALLOC_NO_WATERMARKS)) {
> > > +		warn_alloc_no_wmarks = 0;
> > > +		WARN(1, "Memory reserves are depleted for order:%d, mode:0x%x."
> > > +			" You might consider increasing min_free_kbytes\n",
> > > +			order, gfp_mask);
> > > +	}
> > >  	return NULL;
> > >  }
> > >  
> > 
> > Doesn't this warn for high-order allocations prior to the first call to 
> > direct compaction whereas min_free_kbytes may be irrelevant?
> 
> Hmm, you are concerned about high order ALLOC_NO_WATERMARKS allocation
> which happen prior to compaction, right? I am wondering whether there
> are reasonable chances that a compaction would make a difference if we
> are so depleted that there is no single page with >= order.
> ALLOC_NO_WATERMARKS with high order allocations should be rare if
> existing at all.
> 

No, I'm concerned about get_page_from_freelist() failing for an order-9 
allocation due to _fragmentation_ and then emitting this warning although 
free watermarks may be gigabytes of memory higher than min watermarks.

> > Providing 
> > the order is good, but there's no indication when min_free_kbytes may be 
> > helpful from this warning. 
> 
> I am not sure I understand what you mean here.
> 

You show the order of the failed allocation in your new warning.  Good.  
It won't help to raise min_free_kbytes to infinity if the high-order 
allocation failed due to fragmentation.  Does that make sense?

> > WARN() isn't even going to show the state of memory.
> 
> I was considering to do that but it would make the code unnecessarily
> more complex. If the allocation is allowed to fail it would dump the
> allocation failure. The purpose of the message is to tell us that
> reserves are not sufficient. I am not sure seeing the memory state dump
> would help us much more.
> 

If the purpsoe of the message is to tell us when reserves are 
insufficient, it doesn't achieve that purpose if allocations fail due to 
fragmentation or lowmem_reserve_ratio.
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