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

Dave Hansen
2006-09-01T18:42:05Z

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