xen-blkback: don't store dev_bus_addr

dev_bus_addr returned in the grant ref map operation is the mfn of the
passed page, there's no need to store it in the persistent grant
entry, since we can always get it provided that we have the page.

This reduces the memory overhead of persistent grants in blkback.

While at it, rename the 'seg[i].buf' to be 'seg[i].offset' as
it makes much more sense - as we use that value in bio_add_page
which as the fourth argument expects the offset.

We hadn't used the physical address as part of this at all.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xen.org
[v1: s/buf/offset/]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -442,7 +442,7 @@ int xen_blkif_schedule(void *arg)
 }
 
 struct seg_buf {
-	unsigned long buf;
+	unsigned int offset;
 	unsigned int nsec;
 };
 /*
@@ -621,30 +621,21 @@ static int xen_blkbk_map(struct blkif_request *req,
 				 * If this is a new persistent grant
 				 * save the handler
 				 */
-				persistent_gnts[i]->handle = map[j].handle;
-				persistent_gnts[i]->dev_bus_addr =
-					map[j++].dev_bus_addr;
+				persistent_gnts[i]->handle = map[j++].handle;
 			}
 			pending_handle(pending_req, i) =
 				persistent_gnts[i]->handle;
 
 			if (ret)
 				continue;
-
-			seg[i].buf = persistent_gnts[i]->dev_bus_addr |
-				(req->u.rw.seg[i].first_sect << 9);
 		} else {
-			pending_handle(pending_req, i) = map[j].handle;
+			pending_handle(pending_req, i) = map[j++].handle;
 			bitmap_set(pending_req->unmap_seg, i, 1);
 
-			if (ret) {
-				j++;
+			if (ret)
 				continue;
-			}
-
-			seg[i].buf = map[j++].dev_bus_addr |
-				(req->u.rw.seg[i].first_sect << 9);
 		}
+		seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
 	}
 	return ret;
 }
@@ -971,7 +962,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		       (bio_add_page(bio,
 				     pages[i],
 				     seg[i].nsec << 9,
-				     seg[i].buf & ~PAGE_MASK) == 0)) {
+				     seg[i].offset) == 0)) {
 
 			bio = bio_alloc(GFP_KERNEL, nseg-i);
 			if (unlikely(bio == NULL))

drivers/block/xen-blkback/common.h

@@ -187,7 +187,6 @@ struct persistent_gnt {
 	struct page *page;
 	grant_ref_t gnt;
 	grant_handle_t handle;
-	uint64_t dev_bus_addr;
 	struct rb_node node;
 };