frontswap: allow multiple backends

Change frontswap single pointer to a singly linked list of frontswap
implementations.  Update Xen tmem implementation as register no longer
returns anything.

Frontswap only keeps track of a single implementation; any
implementation that registers second (or later) will replace the
previously registered implementation, and gets a pointer to the previous
implementation that the new implementation is expected to pass all
frontswap functions to if it can't handle the function itself.  However
that method doesn't really make much sense, as passing that work on to
every implementation adds unnecessary work to implementations; instead,
frontswap should simply keep a list of all registered implementations
and try each implementation for any function.  Most importantly, neither
of the two currently existing frontswap implementations in the kernel
actually do anything with any previous frontswap implementation that
they replace when registering.

This allows frontswap to successfully manage multiple implementations by
keeping a list of them all.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 8293262401de..e65ef959546c 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -6,16 +6,16 @@
 #include <linux/bitops.h>
 
 struct frontswap_ops {
-	void (*init)(unsigned);
-	int (*store)(unsigned, pgoff_t, struct page *);
-	int (*load)(unsigned, pgoff_t, struct page *);
-	void (*invalidate_page)(unsigned, pgoff_t);
-	void (*invalidate_area)(unsigned);
+	void (*init)(unsigned); /* this swap type was just swapon'ed */
+	int (*store)(unsigned, pgoff_t, struct page *); /* store a page */
+	int (*load)(unsigned, pgoff_t, struct page *); /* load a page */
+	void (*invalidate_page)(unsigned, pgoff_t); /* page no longer needed */
+	void (*invalidate_area)(unsigned); /* swap type just swapoff'ed */
+	struct frontswap_ops *next; /* private pointer to next ops */
 };
 
 extern bool frontswap_enabled;
-extern struct frontswap_ops *
-	frontswap_register_ops(struct frontswap_ops *ops);
+extern void frontswap_register_ops(struct frontswap_ops *ops);
 extern void frontswap_shrink(unsigned long);
 extern unsigned long frontswap_curr_pages(void);
 extern void frontswap_writethrough(bool);