NFS: Fix a race with PG_private and nfs_release_page()

We don't need to set PG_private for readahead pages, since they never get
unlocked while I/O is in progress. However there is a small race in
nfs_readpage_release() whereby the page may be unlocked, and have
PG_private set.

Fix is to have PG_private set only for the case of writes...

Also fix a bug in nfs_clear_page_writeback(): Don't attempt to clear the
radix_tree tag if we've already deleted the radix tree entry.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pagelist.c

@@ -88,7 +88,6 @@ nfs_create_request(struct nfs_open_context *ctx, struct inode *inode,
 	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;
@@ -136,9 +135,11 @@ void nfs_clear_page_writeback(struct nfs_page *req)
 {
 	struct nfs_inode *nfsi = NFS_I(req->wb_context->dentry->d_inode);
 
+	if (req->wb_page != NULL) {
 		spin_lock(&nfsi->req_lock);
 		radix_tree_tag_clear(&nfsi->nfs_page_tree, req->wb_index, NFS_PAGE_TAG_WRITEBACK);
 		spin_unlock(&nfsi->req_lock);
+	}
 	nfs_unlock_request(req);
 }
 
@@ -153,7 +154,6 @@ void nfs_clear_request(struct nfs_page *req)
 {
 	struct page *page = req->wb_page;
 	if (page != NULL) {
-		ClearPagePrivate(page);
 		page_cache_release(page);
 		req->wb_page = NULL;
 	}

fs/nfs/write.c

@@ -429,6 +429,7 @@ static int nfs_inode_add_request(struct inode *inode, struct nfs_page *req)
 		if (nfs_have_delegation(inode, FMODE_WRITE))
 			nfsi->change_attr++;
 	}
+	SetPagePrivate(req->wb_page);
 	nfsi->npages++;
 	atomic_inc(&req->wb_count);
 	return 0;
@@ -445,6 +446,7 @@ static void nfs_inode_remove_request(struct nfs_page *req)
 	BUG_ON (!NFS_WBACK_BUSY(req));
 
 	spin_lock(&nfsi->req_lock);
+	ClearPagePrivate(req->wb_page);
 	radix_tree_delete(&nfsi->nfs_page_tree, req->wb_index);
 	nfsi->npages--;
 	if (!nfsi->npages) {