Re: [PATCH v7 1/4] mm: introduce compaction and migration for virtio ballooned pages

[Rafael Aquini <aquini@redhat.com>]

On Mon, Aug 13, 2012 at 11:26:19AM +0300, Michael S. Tsirkin wrote:
> > +static inline bool movable_balloon_page(struct page *page)
> > +{
> > +	return (page->mapping && page->mapping == balloon_mapping);
> 
> I am guessing this needs smp_read_barrier_depends, and maybe
> ACCESS_ONCE ...
> 

I'm curious about your guessing here. Could you ellaborate it further, please?


> > +#else
> > +static inline bool isolate_balloon_page(struct page *page) { return false; }
> > +static inline void putback_balloon_page(struct page *page) { return false; }
> > +static inline bool movable_balloon_page(struct page *page) { return false; }
> > +#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && CONFIG_COMPACTION */
> > +
> 
> This does mean that only one type of balloon is useable at a time.
> I wonder whether using a flag in address_space structure instead
> is possible ...

This means we are only introducing this feature for virtio_balloon by now.
Despite the flagging address_space stuff is something we surely can look in the
future, I quite didn't get how we could be using two different types of balloon
devices at the same time for the same system. Could you ellaborate it a little
more, please?


> > +/* __isolate_lru_page() counterpart for a ballooned page */
> > +bool isolate_balloon_page(struct page *page)
> > +{
> > +	if (WARN_ON(!movable_balloon_page(page)))
> 
> Looks like this actually can happen if the page is leaked
> between previous movable_balloon_page and here.
> 
> > +		return false;

Yes, it surely can happen, and it does not harm to catch it here, print a warn and
return. While testing it, I wasn't lucky to see this small window opening, though.

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