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    SubjectRe: [PATCH v7 1/4] mm: introduce compaction and migration for virtio ballooned pages
    On Tue, Aug 14, 2012 at 02:44:05PM -0300, Rafael Aquini wrote:
    > 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?

    well balloon_mapping can change dynamically.


    I think actually rcu is a good fit here.

    >
    > > > +#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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