block/xen-blkfront: split get_grant in 2

Prepare the code to support 64KB page granularity. The first
implementation will use a full Linux page per indirect and persistent
grant. When non-persistent grant is used, each page of a bio request
may be split in multiple grant.

Furthermore, the field page of the grant structure is only used to copy
data from persistent grant or indirect grant. Avoid to set it for other
use case as it will have no meaning given the page will be split in
multiple grant.

Provide 2 functions, to setup indirect grant, the other for bio page.
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/block/xen-blkfront.c

@@ -244,34 +244,77 @@ static int fill_grant_buffer(struct blkfront_info *info, int num)
 	return -ENOMEM;
 }
 
-static struct grant *get_grant(grant_ref_t *gref_head,
-			       struct page *page,
-			       struct blkfront_info *info)
+static struct grant *get_free_grant(struct blkfront_info *info)
 {
 	struct grant *gnt_list_entry;
-	unsigned long buffer_gfn;
 
 	BUG_ON(list_empty(&info->grants));
 	gnt_list_entry = list_first_entry(&info->grants, struct grant,
-	                                  node);
+					  node);
 	list_del(&gnt_list_entry->node);
 
-	if (gnt_list_entry->gref != GRANT_INVALID_REF) {
+	if (gnt_list_entry->gref != GRANT_INVALID_REF)
 		info->persistent_gnts_c--;
+
+	return gnt_list_entry;
+}
+
+static inline void grant_foreign_access(const struct grant *gnt_list_entry,
+					const struct blkfront_info *info)
+{
+	gnttab_page_grant_foreign_access_ref_one(gnt_list_entry->gref,
+						 info->xbdev->otherend_id,
+						 gnt_list_entry->page,
+						 0);
+}
+
+static struct grant *get_grant(grant_ref_t *gref_head,
+			       unsigned long gfn,
+			       struct blkfront_info *info)
+{
+	struct grant *gnt_list_entry = get_free_grant(info);
+
+	if (gnt_list_entry->gref != GRANT_INVALID_REF)
 		return gnt_list_entry;
+
+	/* Assign a gref to this page */
+	gnt_list_entry->gref = gnttab_claim_grant_reference(gref_head);
+	BUG_ON(gnt_list_entry->gref == -ENOSPC);
+	if (info->feature_persistent)
+		grant_foreign_access(gnt_list_entry, info);
+	else {
+		/* Grant access to the GFN passed by the caller */
+		gnttab_grant_foreign_access_ref(gnt_list_entry->gref,
+						info->xbdev->otherend_id,
+						gfn, 0);
 	}
 
+	return gnt_list_entry;
+}
+
+static struct grant *get_indirect_grant(grant_ref_t *gref_head,
+					struct blkfront_info *info)
+{
+	struct grant *gnt_list_entry = get_free_grant(info);
+
+	if (gnt_list_entry->gref != GRANT_INVALID_REF)
+		return gnt_list_entry;
+
 	/* Assign a gref to this page */
 	gnt_list_entry->gref = gnttab_claim_grant_reference(gref_head);
 	BUG_ON(gnt_list_entry->gref == -ENOSPC);
 	if (!info->feature_persistent) {
-		BUG_ON(!page);
-		gnt_list_entry->page = page;
+		struct page *indirect_page;
+
+		/* Fetch a pre-allocated page to use for indirect grefs */
+		BUG_ON(list_empty(&info->indirect_pages));
+		indirect_page = list_first_entry(&info->indirect_pages,
+						 struct page, lru);
+		list_del(&indirect_page->lru);
+		gnt_list_entry->page = indirect_page;
 	}
-	buffer_gfn = xen_page_to_gfn(gnt_list_entry->page);
-	gnttab_grant_foreign_access_ref(gnt_list_entry->gref,
-	                                info->xbdev->otherend_id,
-	                                buffer_gfn, 0);
+	grant_foreign_access(gnt_list_entry, info);
+
 	return gnt_list_entry;
 }
 
@@ -524,32 +567,19 @@ static int blkif_queue_rw_req(struct request *req)
 
 		if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
 		    (i % SEGS_PER_INDIRECT_FRAME == 0)) {
-			struct page *uninitialized_var(page);
-
 			if (segments)
 				kunmap_atomic(segments);
 
 			n = i / SEGS_PER_INDIRECT_FRAME;
-			if (!info->feature_persistent) {
-				struct page *indirect_page;
-
-				/*
-				 * Fetch a pre-allocated page to use for
-				 * indirect grefs
-				 */
-				BUG_ON(list_empty(&info->indirect_pages));
-				indirect_page = list_first_entry(&info->indirect_pages,
-								 struct page, lru);
-				list_del(&indirect_page->lru);
-				page = indirect_page;
-			}
-			gnt_list_entry = get_grant(&gref_head, page, info);
+			gnt_list_entry = get_indirect_grant(&gref_head, info);
 			info->shadow[id].indirect_grants[n] = gnt_list_entry;
 			segments = kmap_atomic(gnt_list_entry->page);
 			ring_req->u.indirect.indirect_grefs[n] = gnt_list_entry->gref;
 		}
 
-		gnt_list_entry = get_grant(&gref_head, sg_page(sg), info);
+		gnt_list_entry = get_grant(&gref_head,
+					   xen_page_to_gfn(sg_page(sg)),
+					   info);
 		ref = gnt_list_entry->gref;
 
 		info->shadow[id].grants_used[i] = gnt_list_entry;