On Mon, Aug 13, 2012 at 11:26:19AM +0300, Michael S. Tsirkin wrote:

quoted

+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?

quoted

+#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?

quoted

+/* __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.

quoted

+		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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