Revert "fuse: move initialization of fuse_file to fuse_writepages() instead of in callback"

This reverts commit 672c3b7.

fuse_writepages() might be called with no dirty pages after all writable
opens were closed.  In this case __fuse_write_file_get() will return NULL
which will trigger the WARNING.

The exact conditions under which this is triggered is unclear and syzbot
didn't find a reproducer yet.

Reported-by: syzbot+217a976dc26ef2fa8711@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/CAJnrk1aQwfvb51wQ5rUSf9N8j1hArTFeSkHqC_3T-mU6_BCD=A@mail.gmail.com/
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: Miklos Szeredi

fs/fuse/file.c

@@ -2288,6 +2288,13 @@ static int fuse_writepages_fill(struct folio *folio,
 	struct folio *tmp_folio;
 	int err;
 
+	if (!data->ff) {
+		err = -EIO;
+		data->ff = fuse_write_file_get(fi);
+		if (!data->ff)
+			goto out_unlock;
+	}
+
 	if (wpa && fuse_writepage_need_send(fc, &folio->page, ap, data)) {
 		fuse_writepages_send(data);
 		data->wpa = NULL;
@@ -2351,23 +2358,21 @@ static int fuse_writepages(struct address_space *mapping,
 			   struct writeback_control *wbc)
 {
 	struct inode *inode = mapping->host;
-	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct fuse_fill_wb_data data;
 	int err;
 
+	err = -EIO;
 	if (fuse_is_bad(inode))
-		return -EIO;
+		goto out;
 
 	if (wbc->sync_mode == WB_SYNC_NONE &&
 	    fc->num_background >= fc->congestion_threshold)
 		return 0;
 
 	data.inode = inode;
 	data.wpa = NULL;
-	data.ff = fuse_write_file_get(fi);
-	if (!data.ff)
-		return -EIO;
+	data.ff = NULL;
 
 	err = -ENOMEM;
 	data.orig_pages = kcalloc(fc->max_pages,
@@ -2381,10 +2386,11 @@ static int fuse_writepages(struct address_space *mapping,
 		WARN_ON(!data.wpa->ia.ap.num_pages);
 		fuse_writepages_send(&data);
 	}
+	if (data.ff)
+		fuse_file_put(data.ff, false);
 
 	kfree(data.orig_pages);
 out:
-	fuse_file_put(data.ff, false);
 	return err;
 }