linux-kernel - Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

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Date:	Wed, 22 Aug 2012 23:19:04 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v8 1/5] mm: introduce a common interface for balloon
 pages mobility

On Wed, Aug 22, 2012 at 12:33:17PM +0300, Michael S. Tsirkin wrote:
> Hmm, so this will busy wait which is unelegant.
> We need some event IMO.

No, it does not busy wait. leak_balloon() is mutual exclusive with migration
steps, so for the case we have one racing against the other, we really want
leak_balloon() dropping the mutex temporarily to allow migration complete its
work of refilling vb->pages list. Also, leak_balloon() calls tell_host(), which
will potentially make it to schedule for each round of vb->pfns leak_balloon()
will release. So, when remove_common() calls leak_balloon() looping on
vb->num_pages, that won't become a tight loop. 
The scheme was apparently working before this series, and it will remain working
after it.

> Also, reading num_pages without a lock here
> which seems wrong.

I'll protect it with vb->balloon_lock mutex. That will be consistent with the
lock protection scheme this patch is introducing for struct virtio_balloon
elements.

> A similar concern applies to normal leaking
> of the balloon: here we might leak less than
> required, then wait for the next config change
> event.

Just as before, same thing here. If you leaked less than required, balloon()
will keep calling leak_balloon() until the balloon target is reached. This
scheme was working before, and it will keep working after this patch.

> How about we signal config_change
> event when pages are back to pages_list?

I really don't know what to tell you here, but, to me, it seems like an
overcomplication that isn't directly entangled with this patch purposes.
Besides, you cannot expect compation / migration happening and racing against
leak_balloon() all the time to make them signal events to the later, so we might
just be creating a wait-forever condition for leak_balloon(), IMHO.

Cheers!

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