linux-kernel - Re: [PATCH v10 1/5] mm: introduce a common interface for balloon pages mobility

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Date:	Tue, 25 Sep 2012 11:00:55 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v10 1/5] mm: introduce a common interface for balloon
 pages mobility

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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