[net/9p] Small non-IO PDUs for zero-copy supporting transports.

If a transport prefers payload to be sent separate from the PDU
(P9_TRANS_PREF_PAYLOAD_SEP), there is no need to allocate msize
PDU buffers(struct p9_fcall).

This patch allocates only upto 4k buffers for this kind of transports
and there won't be any change to the legacy transports.

Hence, this patch on top of zero copy changes allows user to
specify higher msizes through the mount option
without hogging the kernel heap.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

include/net/9p/9p.h

@@ -688,7 +688,7 @@ struct p9_rwstat {
  * @id: protocol operating identifier of type &p9_msg_t
  * @tag: transaction id of the request
  * @offset: used by marshalling routines to track currentposition in buffer
- * @capacity: used by marshalling routines to track total capacity
+ * @capacity: used by marshalling routines to track total malloc'd capacity
  * @pubuf: Payload user buffer given by the caller
  * @pubuf: Payload kernel buffer given by the caller
  * @pbuf_size: pubuf/pkbuf(only one will be !NULL) size to be read/write.

net/9p/client.c

@@ -229,10 +229,23 @@ static struct p9_req_t *p9_tag_alloc(struct p9_client *c, u16 tag)
 			return ERR_PTR(-ENOMEM);
 		}
 		init_waitqueue_head(req->wq);
-		req->tc = kmalloc(sizeof(struct p9_fcall)+c->msize,
-								GFP_KERNEL);
-		req->rc = kmalloc(sizeof(struct p9_fcall)+c->msize,
-								GFP_KERNEL);
+		if ((c->trans_mod->pref & P9_TRANS_PREF_PAYLOAD_MASK) ==
+				P9_TRANS_PREF_PAYLOAD_SEP) {
+			int alloc_msize = min(c->msize, 4096);
+			req->tc = kmalloc(sizeof(struct p9_fcall)+alloc_msize,
+					GFP_KERNEL);
+			req->tc->capacity = alloc_msize;
+			req->rc = kmalloc(sizeof(struct p9_fcall)+alloc_msize,
+					GFP_KERNEL);
+			req->rc->capacity = alloc_msize;
+		} else {
+			req->tc = kmalloc(sizeof(struct p9_fcall)+c->msize,
+					GFP_KERNEL);
+			req->tc->capacity = c->msize;
+			req->rc = kmalloc(sizeof(struct p9_fcall)+c->msize,
+					GFP_KERNEL);
+			req->rc->capacity = c->msize;
+		}
 		if ((!req->tc) || (!req->rc)) {
 			printk(KERN_ERR "Couldn't grow tag array\n");
 			kfree(req->tc);
@@ -243,9 +256,7 @@ static struct p9_req_t *p9_tag_alloc(struct p9_client *c, u16 tag)
 			return ERR_PTR(-ENOMEM);
 		}
 		req->tc->sdata = (char *) req->tc + sizeof(struct p9_fcall);
-		req->tc->capacity = c->msize;
 		req->rc->sdata = (char *) req->rc + sizeof(struct p9_fcall);
-		req->rc->capacity = c->msize;
 	}
 
 	p9pdu_reset(req->tc);