libceph: get rid of zero_page_address

There's not a lot of benefit to zero_page_address, which basically
holds a mapping of the zero page through the life of the messenger
module.  Even with our own mapping, the sendpage interface where
it's used may need to kmap() it again.  It's almost certain to
be in low memory anyway.

So stop treating the zero page specially in write_partial_msg_pages()
and just get rid of zero_page_address entirely.
Signed-off-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>

net/ceph/messenger.c

@@ -61,7 +61,6 @@ static char addr_str[ADDR_STR_COUNT][MAX_ADDR_STR_LEN];
 static atomic_t addr_str_seq = ATOMIC_INIT(0);
 
 static struct page *zero_page;		/* used in certain error cases */
-static void *zero_page_address;		/* kernel virtual addr of zero_page */
 
 const char *ceph_pr_addr(const struct sockaddr_storage *ss)
 {
@@ -111,9 +110,6 @@ void _ceph_msgr_exit(void)
 		ceph_msgr_wq = NULL;
 	}
 
-	BUG_ON(zero_page_address == NULL);
-	zero_page_address = NULL;
-
 	BUG_ON(zero_page == NULL);
 	kunmap(zero_page);
 	page_cache_release(zero_page);
@@ -126,9 +122,6 @@ int ceph_msgr_init(void)
 	zero_page = ZERO_PAGE(0);
 	page_cache_get(zero_page);
 
-	BUG_ON(zero_page_address != NULL);
-	zero_page_address = kmap(zero_page);
-
 	ceph_msgr_wq = alloc_workqueue("ceph-msgr", WQ_NON_REENTRANT, 0);
 	if (ceph_msgr_wq)
 		return 0;
@@ -889,7 +882,7 @@ static int write_partial_msg_pages(struct ceph_connection *con)
 		} else {
 			page = zero_page;
 			if (do_datacrc)
-				kaddr = zero_page_address;
+				kaddr = kmap(page);
 		}
 		len = min_t(int, max_write - con->out_msg_pos.page_pos,
 			    total_max_write);
@@ -908,7 +901,7 @@ static int write_partial_msg_pages(struct ceph_connection *con)
 				      con->out_msg_pos.page_pos + page_shift,
 				      len, 1);
 
-		if (do_datacrc && kaddr != zero_page_address)
+		if (do_datacrc)
 			kunmap(page);
 
 		if (ret <= 0)