handle partial page update of page in cache that is not uptodate better for...

handle partial page update of page in cache that is not uptodate better for the situation in which file is open writeonly
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/file.c

@@ -778,6 +778,9 @@ cifs_writepage(struct page* page, struct writeback_control *wbc)
 	xid = GetXid();
 /* BB add check for wbc flags */
 	page_cache_get(page);
+        if (!PageUptodate(page)) {
+		cFYI(1,("ppw - page not up to date"));
+	}
 	
 	rc = cifs_partialpagewrite(page,0,PAGE_CACHE_SIZE);
 	SetPageUptodate(page); /* BB add check for error and Clearuptodate? */
@@ -795,8 +798,7 @@ cifs_commit_write(struct file *file, struct page *page, unsigned offset,
 	int rc = 0;
 	struct inode *inode = page->mapping->host;
 	loff_t position = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
-	/* struct cifsFileInfo *open_file;
-	struct cifs_sb_info *cifs_sb; */
+	char * page_data;
 
 	xid = GetXid();
 	cFYI(1,("commit write for page %p up to position %lld for %d",page,position,to));
@@ -827,7 +829,31 @@ cifs_commit_write(struct file *file, struct page *page, unsigned offset,
 			cFYI(1,(" SetEOF (commit write) rc = %d",rc));
 		}*/
 	}
-	set_page_dirty(page);
+        if (!PageUptodate(page)) {
+		position =  ((loff_t)page->index << PAGE_CACHE_SHIFT) + offset;
+		/* can not rely on (or let) writepage write this data */
+		if(to < offset) {
+			cFYI(1,("Illegal offsets, can not copy from %d to %d",
+				offset,to));
+			FreeXid(xid);
+			return rc;
+		}
+		/* this is probably better than directly calling 
+		partialpage_write since in this function
+		the file handle is known which we might as well
+		leverage */
+		/* BB check if anything else missing out of ppw */
+		/* such as updating last write time */
+		page_data = kmap(page);
+		rc = cifs_write(file, page_data+offset,to-offset,
+                                        &position);
+		if(rc > 0)
+			rc = 0;
+		/* else if rc < 0 should we set writebehind rc? */
+		kunmap(page);
+	} else {	
+		set_page_dirty(page);
+	}
 
 	FreeXid(xid);
 	return rc;
@@ -2003,10 +2029,14 @@ int cifs_prepare_write(struct file *file, struct page *page,
 			SetPageUptodate(page);
 
 		/* might as well read a page, it is fast enough */
-		rc = cifs_readpage_worker(file,page,&offset);
-		/* if this returns an error should we try using another
+		if((file->f_flags & O_ACCMODE) != O_WRONLY) {
+			rc = cifs_readpage_worker(file,page,&offset);
+		} else {
+		/* should we try using another
 		file handle if there is one - how would we lock it
 		to prevent close of that handle racing with this read? */
+		/* In any case this will be written out by commit_write */
+		}
 	}
 
 	/* BB should we pass any errors back? e.g. if we do not have read access to the file */