[PATCH 3/3] pnfs: add a new mechanism to select a layout driver according to an ordered list

[Jeff Layton <jlayton@poochiereds.net>]

Currently, the layout driver selection code attempts to divine meaning
from the order of the layout driver list sent by the server.
Unfortunately, the spec doesn't place any significance on the order
and the server is free to send them in any order it likes.

Instead, set a list of preferred driver types in the kernel and have
the selection code try them in order until it finds one that can be
loaded.

If we go through the whole list of preferred drivers and don't find one,
then try any that weren't recognized in the first scan. This should
allow the use of 3rd party and experimental drivers without needing to
muck with the order of preference.

For now, the order of preference is hardcoded, but it should be possible
to make this configurable (via module param perhaps?).

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
---
 fs/nfs/pnfs.c | 71 +++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 50 insertions(+), 21 deletions(-)

diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index b02cad9c04bf..3ec5f2b392b6 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -99,6 +99,21 @@ unset_pnfs_layoutdriver(struct nfs_server *nfss)
 }
 
 /*
+ * When the server sends a list of layout types, we choose one in the order
+ * given in the list below.
+ *
+ * FIXME: should this list be configurable via module_param or something?
+ */
+static const u32 ld_prefs[] = {
+	LAYOUT_SCSI,
+	LAYOUT_BLOCK_VOLUME,
+	LAYOUT_OSD2_OBJECTS,
+	LAYOUT_FLEX_FILES,
+	LAYOUT_NFSV4_1_FILES,
+	0
+};
+
+/*
  * Try to set the server's pnfs module to the pnfs layout type specified by id.
  * Currently only one pNFS layout driver per filesystem is supported.
  *
@@ -110,7 +125,7 @@ set_pnfs_layoutdriver(struct nfs_server *server, const struct nfs_fh *mntfh,
 {
 	struct pnfs_layoutdriver_type *ld_type = NULL;
 	u32 id;
-	int i;
+	int i, j;
 
 	if (!(server->nfs_client->cl_exchange_flags &
 		 (EXCHGID4_FLAG_USE_NON_PNFS | EXCHGID4_FLAG_USE_PNFS_MDS))) {
@@ -118,31 +133,45 @@ set_pnfs_layoutdriver(struct nfs_server *server, const struct nfs_fh *mntfh,
 			__func__, server->nfs_client->cl_exchange_flags);
 		goto out_no_driver;
 	}
-	/*
-	 * If server supports more than one layout types.
-	 * By assuming, that server will put 'common default' as the first
-	 * entry, try all following entries ibefore and fall back to the default
-	 * if we did not found a matching one.
-	 */
-	for(i = 1; i < NFS_MAX_LAYOUT_TYPES && ids[i] != 0; i++) {
-		id = ids[i];
-		request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX, id);
-		ld_type = find_pnfs_driver(id);
-		if(ld_type)
-			goto found_module;
 
-		dprintk("%s: No pNFS module found for %u.\n", __func__, id);
+	/* scan the list for each layout type in order of preference */
+	for (j = 0; ld_prefs[j] != 0; j++) {
+		for (i = 0; i < NFS_MAX_LAYOUT_TYPES && ids[i] != 0; i++) {
+			id = ids[i];
+
+			if (ld_prefs[j] == id) {
+				request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX, id);
+				ld_type = find_pnfs_driver(id);
+				if (ld_type)
+					goto found_module;
+			}
+		}
 	}
 
-	/*
-	 * no other layout types found. Try default one.
-	 */
-	id = ids[0];
-	request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX, id);
-	ld_type = find_pnfs_driver(id);
+	/* didn't find one -- make sure we try any that weren't in ld_prefs */
+	for (i = 0; i < NFS_MAX_LAYOUT_TYPES && ids[i] != 0; i++) {
+		bool	match = false;
+
+		id = ids[i];
+
+		/* Was it in the prefs list? */
+		for (j = 0; ld_prefs[j] != 0; j++) {
+			if (ld_prefs[j] != id) {
+				match = true;
+				break;
+			}
+		}
+
+		if (!match) {
+			request_module("%s-%u", LAYOUT_NFSV4_1_MODULE_PREFIX, id);
+			ld_type = find_pnfs_driver(id);
+			if (ld_type)
+				goto found_module;
+		}
+	}
 
 	if (!ld_type) {
-		dprintk("%s: No pNFS module found for %u.\n", __func__, id);
+		dprintk("%s: No pNFS module found!\n", __func__);
 		goto out_no_driver;
 	}
 
-- 
2.5.5