linux-kernel - Re: [patch 3/9] Guest page hinting: volatile page cache.

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Date:	Fri, 01 Sep 2006 11:41:46 -0700
From:	Dave Hansen <haveblue@...ibm.com>
To:	schwidefsky@...ibm.com
Cc:	Andy Whitcroft <apw@...dowen.org>, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, akpm@...l.org,
	nickpiggin@...oo.com.au, frankeh@...son.ibm.com
Subject: Re: [patch 3/9] Guest page hinting: volatile page cache.

On Fri, 2006-09-01 at 20:31 +0200, Martin Schwidefsky wrote:
> On Fri, 2006-09-01 at 11:23 -0700, Dave Hansen wrote:
> > OK, and there's no other workable solution to exclude each other from
> > running at the same time than a bit in page->flags?
> > 
> > It seems like that hashed lock (or lock in mem_map[]) we were talking
> > about earlier might be applicable here, too.
> 
> The indication which page has already been removed from the page cache
> by a discard fault is by definition per-page.

Right.  So having a single bit that was set and cleared wouldn't work
because it could get interpreted incorrectly for multiple pages.  But,
what about a lock?

> The situation is different
> compared to the one with PG_state_change which is used to protect
> critical sections. After the cpu left the critical section the bit can
> be clear again. The discard bit cannot be cleared until the page really
> has been freed.

While something like the following wouldn't be scalable, it would
functionally work, right?

+static void __page_discard(struct page *page)
+{
+	spin_lock(discard_lock);
...
+	spin_unlock(discard_lock);
+}

+void __delete_from_swap_cache(struct page *page)
+{
+	spin_lock(discard_lock);
...
+	spin_unlock(discard_lock);
+}

+void __remove_from_page_cache(struct page *page)
+{
+	spin_lock(discard_lock);
...
+	spin_unlock(discard_lock);
+}


-- Dave

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