Re: [PATCH 2/2] AFS: Implement shared-writable mmap

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David Howells wrote:
Implement shared-writable mmap for AFS.

The key with which to access the file is obtained from the VMA at the point
where the PTE is made writable by the page_mkwrite() VMA op and cached in the
affected page.

If there's an outstanding write on the page made with a different key, then
page_mkwrite() will flush it before attaching a record of the new key.

Good, will be nice to get a page_mkwrite() user in the tree.



+/*
+ * notification that a previously read-only page is about to become writable
+ * - if it returns an error, the caller will deliver a bus error signal
+ *
+ * we use this to make a record of the key with which the writeback should be
+ * performed and to flush any outstanding writes made with a different key
+ *
+ * the key to be used is attached to the file pinned by the VMA
+ */
+int afs_page_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+	struct afs_vnode *vnode = AFS_FS_I(vma->vm_file->f_mapping->host);
+	struct key *key = vma->vm_file->private_data;
+	int ret;
+
+	_enter("{{%x:%u},%x},{%lx}",
+	       vnode->fid.vid, vnode->fid.vnode, key_serial(key), page->index);
+
+	lock_page(page);
+	ret = afs_prepare_write(vma->vm_file, page, 0, 0);
+	unlock_page(page);
+
+	_leave(" = %d", ret);
+	return ret;
+}

By the looks of afs_prepare_write, it is going to go bang when the page
gets truncated before lock_page.

Checking page->mapping after lock_page should do the trick.

--
SUSE Labs, Novell Inc.
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