xen-blkback: use balloon pages for all mappings

Using balloon pages for all granted pages allows us to simplify the
logic in blkback, especially in the xen_blkbk_map function, since now
we can decide if we want to map a grant persistently or not after we
have actually mapped it. This could not be done before because
persistent grants used ballooned pages, whereas non-persistent grants
used pages from the kernel.

This patch also introduces several changes, the first one is that the
list of free pages is no longer global, now each blkback instance has
it's own list of free pages that can be used to map grants. Also, a
run time parameter (max_buffer_pages) has been added in order to tune
the maximum number of free pages each blkback instance will keep in
it's buffer.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: xen-devel@lists.xen.org
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/ABI/stable/sysfs-bus-xen-backend

@@ -73,3 +73,11 @@ KernelVersion:	3.0
 Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
 Description:
                 Number of sectors written by the frontend.
+
+What:           /sys/module/xen_blkback/parameters/max_buffer_pages
+Date:           March 2013
+KernelVersion:  3.10
+Contact:        Roger Pau Monné <roger.pau@citrix.com>
+Description:
+                Maximum number of free pages to keep in each block
+                backend buffer.

drivers/block/xen-blkback/common.h

@@ -220,6 +220,11 @@ struct xen_blkif {
 	struct rb_root		persistent_gnts;
 	unsigned int		persistent_gnt_c;
 
+	/* buffer of free pages to map grant refs */
+	spinlock_t		free_pages_lock;
+	int			free_pages_num;
+	struct list_head	free_pages;
+
 	/* statistics */
 	unsigned long		st_print;
 	unsigned long long			st_rd_req;

drivers/block/xen-blkback/xenbus.c

@@ -118,6 +118,9 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	blkif->st_print = jiffies;
 	init_waitqueue_head(&blkif->waiting_to_free);
 	blkif->persistent_gnts.rb_node = NULL;
+	spin_lock_init(&blkif->free_pages_lock);
+	INIT_LIST_HEAD(&blkif->free_pages);
+	blkif->free_pages_num = 0;
 
 	return blkif;
 }