cachefiles: Preset the state xattr when creating a new file

With a really small cache, cachefiles is likely to see a lot of writes
hitting ENOSPC - and this can include setxattr that sets the state xattr on
a cachefile - but we don't really want to successfully fill a cache file
only to have to scrap it because we can't set the xattr.

Instead, preset the xattr when we create the tmpfile we're going to use,
and scrap the file at that point if we get ENOSPC.  Only if setxattr
succeeds do we allow data to be written to the file.

Note that there is a potential performance loss in that writes to the cache
have to be delayed until this is completed - but we do the tmpfile/setxattr
in parallel, starting when the file is opened and only have to wait once
writeback occurs.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara <pc@manguebit.org>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org

fs/cachefiles/internal.h

diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index b260511..664be64 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -283,6 +283,7 @@ void cachefiles_withdraw_volume(struct cachefiles_volume *volume);
 /*
  * xattr.c
  */
+int cachefiles_preset_object_xattr(struct cachefiles_object *object, struct file *file);
 extern int cachefiles_set_object_xattr(struct cachefiles_object *object);
 extern int cachefiles_check_auxdata(struct cachefiles_object *object,
 				    struct file *file);

fs/cachefiles/namei.c

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index ca09384..ef656a3 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -450,6 +450,11 @@ struct file *cachefiles_create_tmpfile(struct cachefiles_object *object)
 		pr_notice("Cache does not support read_iter and write_iter\n");
 		goto err_unuse;
 	}
+
+	/* Preallocate space for the xattr. */
+	ret = cachefiles_preset_object_xattr(object, file);
+	if (ret < 0)
+		goto err_unuse;
 out:
 	cachefiles_end_secure(cache, saved_cred);
 	object->content_info = CACHEFILES_CONTENT_ALL;

fs/cachefiles/xattr.c

diff --git a/fs/cachefiles/xattr.c b/fs/cachefiles/xattr.c
index 551a3b0..068e2c1 100644
--- a/fs/cachefiles/xattr.c
+++ b/fs/cachefiles/xattr.c
@@ -35,6 +35,57 @@ struct cachefiles_vol_xattr {
 } __packed;
 
 /*
+ * Preset the state xattr on a cache file to allocate space for it.
+ */
+int cachefiles_preset_object_xattr(struct cachefiles_object *object, struct file *file)
+{
+	struct cachefiles_xattr *buf;
+	struct dentry *dentry = file->f_path.dentry;
+	unsigned int len = object->cookie->aux_len;
+	int ret;
+
+	buf = kzalloc(sizeof(struct cachefiles_xattr) + max(len, sizeof(__be64)), GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	buf->type		= CACHEFILES_COOKIE_TYPE_DATA;
+	buf->content		= CACHEFILES_CONTENT_DIRTY;
+
+	ret = cachefiles_inject_write_error();
+	if (ret == 0) {
+		ret = mnt_want_write_file(file);
+		if (ret == 0) {
+			ret = vfs_setxattr(&nop_mnt_idmap, dentry,
+					   cachefiles_xattr_cache, buf,
+					   sizeof(struct cachefiles_xattr) + len, 0);
+			mnt_drop_write_file(file);
+		}
+	}
+	if (ret < 0) {
+		trace_cachefiles_vfs_error(object, file_inode(file), ret,
+					   cachefiles_trace_setxattr_error);
+		trace_cachefiles_coherency(object, file_inode(file)->i_ino,
+					   object->object_size,
+					   buf->data, buf->content,
+					   cachefiles_coherency_set_fail);
+		switch (ret) {
+		case -ENOMEM:
+		case -ENOSPC:
+			break;
+		default:
+			cachefiles_io_error_obj(
+				object,
+				"Failed to set xattr with error %d", ret);
+			break;
+		}
+	}
+
+	kfree(buf);
+	_leave(" = %d", ret);
+	return ret;
+}
+
+/*
  * set the state xattr on a cache file
  */
 int cachefiles_set_object_xattr(struct cachefiles_object *object)
@@ -82,10 +133,16 @@ int cachefiles_set_object_xattr(struct cachefiles_object *object)
 		trace_cachefiles_coherency(object, file_inode(file)->i_ino,
 					   object_size, buf->data, buf->content,
 					   cachefiles_coherency_set_fail);
-		if (ret != -ENOMEM)
+		switch (ret) {
+		case -ENOMEM:
+			break;
+		case -ENOSPC:
+		default:
 			cachefiles_io_error_obj(
 				object,
 				"Failed to set xattr with error %d", ret);
+			break;
+		}
 	} else {
 		trace_cachefiles_coherency(object, file_inode(file)->i_ino,
 					   object_size, buf->data, buf->content,