cifs: pick channels for individual subrequests

The netfs library could break down a read request into
multiple subrequests. When multichannel is used, there is
potential to improve performance when each of these
subrequests pick a different channel.

Today we call cifs_pick_channel when the main read request
is initialized in cifs_init_request. This change moves this to
cifs_prepare_read, which is the right place to pick channel since
it gets called for each subrequest.

Interestingly cifs_prepare_write already does channel selection
for individual subreq, but looks like it was missed for read.
This is especially important when multichannel is used with
increased rasize.

In my test setup, with rasize set to 8MB, a sequential read
of large file was taking 11.5s without this change. With the
change, it completed in 9s. The difference is even more signigicant
with bigger rasize.

Cc: <[email protected]>
Cc: David Howells <[email protected]>
Signed-off-by: Shyam Prasad N <[email protected]>
Signed-off-by: Steve French <[email protected]>

Author: sprasad-microsoft

fs/smb/client/cifsglob.h

@@ -1508,7 +1508,6 @@ struct cifs_io_parms {
 struct cifs_io_request {
 	struct netfs_io_request		rreq;
 	struct cifsFileInfo		*cfile;
-	struct TCP_Server_Info		*server;
 	pid_t				pid;
 };
 

fs/smb/client/file.c

@@ -147,7 +147,7 @@ static int cifs_prepare_read(struct netfs_io_subrequest *subreq)
 	struct netfs_io_request *rreq = subreq->rreq;
 	struct cifs_io_subrequest *rdata = container_of(subreq, struct cifs_io_subrequest, subreq);
 	struct cifs_io_request *req = container_of(subreq->rreq, struct cifs_io_request, rreq);
-	struct TCP_Server_Info *server = req->server;
+	struct TCP_Server_Info *server;
 	struct cifs_sb_info *cifs_sb = CIFS_SB(rreq->inode->i_sb);
 	size_t size;
 	int rc = 0;
@@ -156,6 +156,8 @@ static int cifs_prepare_read(struct netfs_io_subrequest *subreq)
 		rdata->xid = get_xid();
 		rdata->have_xid = true;
 	}
+
+	server = cifs_pick_channel(tlink_tcon(req->cfile->tlink)->ses);
 	rdata->server = server;
 
 	if (cifs_sb->ctx->rsize == 0)
@@ -198,7 +200,7 @@ static void cifs_issue_read(struct netfs_io_subrequest *subreq)
 	struct netfs_io_request *rreq = subreq->rreq;
 	struct cifs_io_subrequest *rdata = container_of(subreq, struct cifs_io_subrequest, subreq);
 	struct cifs_io_request *req = container_of(subreq->rreq, struct cifs_io_request, rreq);
-	struct TCP_Server_Info *server = req->server;
+	struct TCP_Server_Info *server = rdata->server;
 	int rc = 0;
 
 	cifs_dbg(FYI, "%s: op=%08x[%x] mapping=%p len=%zu/%zu\n",
@@ -266,7 +268,6 @@ static int cifs_init_request(struct netfs_io_request *rreq, struct file *file)
 		open_file = file->private_data;
 		rreq->netfs_priv = file->private_data;
 		req->cfile = cifsFileInfo_get(open_file);
-		req->server = cifs_pick_channel(tlink_tcon(req->cfile->tlink)->ses);
 		if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_RWPIDFORWARD)
 			req->pid = req->cfile->pid;
 	} else if (rreq->origin != NETFS_WRITEBACK) {