Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Aug 15, 2012 at 01:01:08PM +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 15, 2012 at 10:48:39AM +0100, Mel Gorman wrote:
> > On Wed, Aug 15, 2012 at 12:25:28PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, Aug 15, 2012 at 10:05:28AM +0100, Mel Gorman wrote:
> > > > On Tue, Aug 14, 2012 at 05:11:13PM -0300, Rafael Aquini wrote:
> > > > > On Tue, Aug 14, 2012 at 10:51:39PM +0300, Michael S. Tsirkin wrote:
> > > > > > What I think you should do is use rcu for access.
> > > > > > And here sync rcu before freeing.
> > > > > > Maybe an overkill but at least a documented synchronization
> > > > > > primitive, and it is very light weight.
> > > > > > 
> > > > > 
> > > > > I liked your suggestion on barriers, as well.
> > > > > 
> > > > 
> > > > I have not thought about this as deeply as I shouold but is simply rechecking
> > > > the mapping under the pages_lock to make sure the page is still a balloon
> > > > page an option? i.e. use pages_lock to stabilise page->mapping.
> > > 
> > > To clarify, are you concerned about cost of rcu_read_lock
> > > for non balloon pages?
> > > 
> > 
> > Not as such, but given the choice between introducing RCU locking and
> > rechecking page->mapping under a spinlock I would choose the latter as it
> > is more straight-forward.
> 
> OK but checking it how? page->mapping == balloon_mapping does not scale to
> multiple balloons,

I was thinking of exactly that page->mapping == balloon_mapping check. As I
do not know how many active balloon drivers there might be I cannot guess
in advance how much of a scalability problem it will be.

> so I hoped we can switch to
> page->mapping->flags & BALLOON_MAPPING or some such,
> but this means we dereference it outside the lock ...
> 

That also sounded like future stuff to me that would be justified with
profiling if necessary. Personally I would have started with the spinlock
and a simple check and moved to RCU later when either scalability was a
problem or it was found there was a need to stabilise whether a page was
a balloon page or not outside a spinlock.

This is not a NAK to the idea and I'm not objecting to RCU being used now
if that is what is really desired. I just suspect it's making the series
more complex than it needs to be right now.

-- 
Mel Gorman
SUSE Labs

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