Re: [PATCH v10 1/5] mm: introduce a common interface for balloonpages mobility

[Rafael Aquini]

On Tue, Sep 25, 2012 at 03:05:49AM +0200, Michael S. Tsirkin wrote:
> If these are all under page lock these barriers just confuse things,
> because they are almost never enough by themselves.
> So in that case it would be better to drop them and document
> usage as you are going to.
>

Would the following make more sense (with the proprer comments, as well) ?

---8<---
+static inline void balloon_page_set(struct page *page,
+                                   struct address_space *mapping,
+                                   struct list_head *head)
+{
+       list_add(&page->lru, head);
+       smp_wmb();
+       page->mapping = mapping;
+}
+
+static inline void balloon_page_del(struct page *page)
+{
+       page->mapping = NULL;
+       smp_wmb();
+       list_del(&page->lru);
+}
+
+static inline bool __is_movable_balloon_page(struct page *page)
+{
+       struct address_space *mapping = ACCESS_ONCE(page->mapping);
+       smp_read_barrier_depends();
+       return mapping_balloon(mapping);
+}
+
---8<---

There's still a case where we have to test page->mapping->flags and we cannot
afford to wait for, or grab, the page lock @ isolate_migratepages_range().
The barriers won't avoid leak_ballon() racing against isolate_migratepages_range(),
but they surely will make tests for page->mapping more consistent. And for those
cases where leak_balloon() races against
isolate_migratepages_range->isolate_balloon_page(), we solve the conflict of
interest through page refcounting and page lock. I'm preparing a more extensive
doc to include at Documentation/ to explain the interfaces and how we cope with
these mentioned races, as well.
 
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