linux-kernel - Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher order

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Date:   Wed, 09 Jan 2019 08:09:11 -0800
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Arun KS <arunks@...eaurora.org>
Cc:     arunks.linux@...il.com, akpm@...ux-foundation.org,
        mhocko@...nel.org, vbabka@...e.cz, osalvador@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        getarunks@...il.com
Subject: Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as
 higher order

On Wed, 2019-01-09 at 11:51 +0530, Arun KS wrote:
> On 2019-01-09 03:47, Alexander Duyck wrote:
> > On Fri, 2019-01-04 at 10:31 +0530, Arun KS wrote:
> > > When freeing pages are done with higher order, time spent on 
> > > coalescing
> > > pages by buddy allocator can be reduced.  With section size of 256MB, 
> > > hot
> > > add latency of a single section shows improvement from 50-60 ms to 
> > > less
> > > than 1 ms, hence improving the hot add latency by 60 times.  Modify
> > > external providers of online callback to align with the change.
> > > 
> > > Signed-off-by: Arun KS <arunks@...eaurora.org>
> > > Acked-by: Michal Hocko <mhocko@...e.com>
> > > Reviewed-by: Oscar Salvador <osalvador@...e.de>
> > 
> > Sorry, ended up encountering a couple more things that have me a bit
> > confused.
> > 
> > [...]
> > 
> > > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> > > index 5301fef..211f3fe 100644
> > > --- a/drivers/hv/hv_balloon.c
> > > +++ b/drivers/hv/hv_balloon.c
> > > @@ -771,7 +771,7 @@ static void hv_mem_hot_add(unsigned long start, 
> > > unsigned long size,
> > >  	}
> > >  }
> > > 
> > > -static void hv_online_page(struct page *pg)
> > > +static int hv_online_page(struct page *pg, unsigned int order)
> > >  {
> > >  	struct hv_hotadd_state *has;
> > >  	unsigned long flags;
> > > @@ -783,10 +783,12 @@ static void hv_online_page(struct page *pg)
> > >  		if ((pfn < has->start_pfn) || (pfn >= has->end_pfn))
> > >  			continue;
> > > 
> > > -		hv_page_online_one(has, pg);
> > > +		hv_bring_pgs_online(has, pfn, (1UL << order));
> > >  		break;
> > >  	}
> > >  	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
> > > +
> > > +	return 0;
> > >  }
> > > 
> > >  static int pfn_covered(unsigned long start_pfn, unsigned long 
> > > pfn_cnt)
> > 
> > So the question I have is why was a return value added to these
> > functions? They were previously void types and now they are int. What
> > is the return value expected other than 0?
> 
> Earlier with returning a void there was now way for an arch code to 
> denying onlining of this particular page. By using an int as return 
> type, we can implement this. In one of the boards I was using, there are 
> some pages which should not be onlined because they are used for other 
> purposes(like secure trust zone or hypervisor).

So where is the code using that? I don't see any functions in the
kernel that are returning anything other than 0. Maybe you should hold
off on changing the return type and make that a separate patch to be
enabled when you add the new functions that can return non-zero values.

That way if someone wants to backport this they are just getting the
bits needed to enable the improved hot-plug times without adding the
extra overhead for changing the return type.