+/*
+ * prepare a page for being written to
+ */
+static int afs_prepare_page(struct afs_vnode *vnode, struct page *page,
+ struct key *key, unsigned offset, unsigned to)
+{
+ unsigned eof, tail, start, stop, len;
+ loff_t i_size, pos;
+ void *p;
+ int ret;
+
+ _enter("");
+
+ if (offset == 0 && to == PAGE_SIZE)
+ return 0;
+
+ p = kmap(page);
+
+ i_size = i_size_read(&vnode->vfs_inode);
+ pos = (loff_t) page->index << PAGE_SHIFT;
+ if (pos >= i_size) {
+ /* partial write, page beyond EOF */
+ _debug("beyond");
+ if (offset > 0)
+ memset(p, 0, offset);
+ if (to < PAGE_SIZE)
+ memset(p + to, 0, PAGE_SIZE - to);
+ kunmap(page);
+ return 0;
+ }
+
+ if (i_size - pos >= PAGE_SIZE) {
+ /* partial write, page entirely before EOF */
+ _debug("before");
+ tail = eof = PAGE_SIZE;
+ } else {
+ /* partial write, page overlaps EOF */
+ eof = i_size - pos;
+ _debug("overlap %u", eof);
+ tail = max(eof, to);
+ if (tail < PAGE_SIZE)
+ memset(p + tail, 0, PAGE_SIZE - tail);
+ if (offset > eof)
+ memset(p + eof, 0, PAGE_SIZE - eof);
+ }
+
+ kunmap(p);
+
+ ret = 0;
+ if (offset > 0 || eof > to) {
+ /* need to fill one or two bits that aren't going to be written
+ * (cover both fillers in one read if there are two) */
+ start = (offset > 0) ? 0 : to;
+ stop = (eof > to) ? eof : offset;
+ len = stop - start;
+ _debug("wr=%u-%u av=0-%u rd=%u@%u",
+ offset, to, eof, start, len);
+ ret = afs_fill_page(vnode, key, start, len, page);
+ }
+
+ _leave(" = %d", ret);
+ return ret;
+}
+
+/*
+ * prepare to perform part of a write to a page
+ * - the caller holds the page locked, preventing it from being written out or
+ * modified by anyone else
+ */
+int afs_prepare_write(struct file *file, struct page *page,
+ unsigned offset, unsigned to)
+{
+ struct afs_writeback *candidate, *wb;
+ struct afs_vnode *vnode = AFS_FS_I(file->f_dentry->d_inode);
+ struct key *key = file->private_data;
+ pgoff_t index;
+ int ret;
+
+ _enter("{%x:%u},{%lx},%u,%u",
+ vnode->fid.vid, vnode->fid.vnode, page->index, offset, to);
+
+ candidate = kzalloc(sizeof(*candidate), GFP_KERNEL);
+ if (!candidate)
+ return -ENOMEM;
+ candidate->vnode = vnode;
+ candidate->first = candidate->last = page->index;
+ candidate->offset_first = offset;
+ candidate->to_last = to;
+ candidate->usage = 1;
+ candidate->state = AFS_WBACK_PENDING;
+ init_waitqueue_head(&candidate->waitq);
+
+ if (!PageUptodate(page)) {
+ _debug("not up to date");
+ ret = afs_prepare_page(vnode, page, key, offset, to);
+ if (ret < 0) {
+ kfree(candidate);
+ _leave(" = %d [prep]", ret);
+ return ret;
+ }
+ SetPageUptodate(page);
+ }
