Re: [PATCH 3/3] AFS: Implement basic file write support

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David Howells wrote:

+/*
+ * 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);
+	}


Why do you call SetPageUptodate when the page is not up to date?
That leaks uninitialised data, AFAIKS.

--
SUSE Labs, Novell Inc.
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