MMAP Optimization

Enable zfs_getpage, zfs_fillpage, zfs_putpage, zfs_putapage functions.
The functions have been modified to make them Linux friendly.

ZFS uses these functions to read/write the mmapped pages. Using them
from readpage/writepage results in clear code. The patch also adds
readpages and writepages interface functions to read/write list of
pages in one function call.

The code change handles the first mmap optimization mentioned on
https://github.com/behlendorf/zfs/issues/225

Signed-off-by: Prasad Joshi <pjoshi@stec-inc.com>
Signed-off-by: Brian Behlendorf <behlendorf@llnl.gov>
Issue #255

include/sys/zfs_vnops.h

@@ -71,6 +71,9 @@ extern int zfs_getsecattr(struct inode *ip, vsecattr_t *vsecp, int flag,
     cred_t *cr);
 extern int zfs_setsecattr(struct inode *ip, vsecattr_t *vsecp, int flag,
     cred_t *cr);
+extern int zfs_getpage(struct inode *ip, struct page *pl[], int nr_pages);
+extern int zfs_putpage(struct page *page, struct writeback_control *wbc,
+    void *data);
 
 #ifdef	__cplusplus
 }

include/sys/zpl.h

@@ -29,6 +29,7 @@
 #include <linux/vfs_compat.h>
 #include <linux/xattr_compat.h>
 #include <linux/exportfs.h>
+#include <linux/writeback.h>
 
 /* zpl_inode.c */
 extern const struct inode_operations zpl_inode_operations;