NFS: Avoid races between writebacks and truncation

Currently, there is no serialisation between NFS asynchronous writebacks
and truncation at the page level due to the fact that nfs_sync_inode()
cannot lock the pages that it is about to write out.

This means that it is possible to be flushing out data (and calling something
like set_page_writeback()) while the page cache is busy evicting the page.
Oops...

Use the hooks provided in try_to_release_page() to ensure that dirty pages
are always written back to storage before we evict them.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/file.c

@@ -316,6 +316,17 @@ static int nfs_commit_write(struct file *file, struct page *page, unsigned offse
 	return status;
 }
 
+static int nfs_invalidate_page(struct page *page, unsigned long offset)
+{
+	/* FIXME: we really should cancel any unstarted writes on this page */
+	return 1;
+}
+
+static int nfs_release_page(struct page *page, gfp_t gfp)
+{
+	return !nfs_wb_page(page->mapping->host, page);
+}
+
 struct address_space_operations nfs_file_aops = {
 	.readpage = nfs_readpage,
 	.readpages = nfs_readpages,
@@ -324,6 +335,8 @@ struct address_space_operations nfs_file_aops = {
 	.writepages = nfs_writepages,
 	.prepare_write = nfs_prepare_write,
 	.commit_write = nfs_commit_write,
+	.invalidatepage = nfs_invalidate_page,
+	.releasepage = nfs_release_page,
 #ifdef CONFIG_NFS_DIRECTIO
 	.direct_IO = nfs_direct_IO,
 #endif

fs/nfs/pagelist.c

@@ -85,6 +85,10 @@ nfs_create_request(struct nfs_open_context *ctx, struct inode *inode,
 	atomic_set(&req->wb_complete, 0);
 	req->wb_index	= page->index;
 	page_cache_get(page);
+	BUG_ON(PagePrivate(page));
+	BUG_ON(!PageLocked(page));
+	BUG_ON(page->mapping->host != inode);
+	SetPagePrivate(page);
 	req->wb_offset  = offset;
 	req->wb_pgbase	= offset;
 	req->wb_bytes   = count;
@@ -147,8 +151,10 @@ void nfs_clear_page_writeback(struct nfs_page *req)
  */
 void nfs_clear_request(struct nfs_page *req)
 {
-	if (req->wb_page) {
-		page_cache_release(req->wb_page);
+	struct page *page = req->wb_page;
+	if (page != NULL) {
+		ClearPagePrivate(page);
+		page_cache_release(page);
 		req->wb_page = NULL;
 	}
 }