[RFC PATCH 31/61] cachefiles: Implement new fscache I/O backend API

[David Howells]

Implement the new fscache I/O backend API in cachefiles.  The
cachefiles_object struct carries a non-accounted file to the cachefiles
object (so that it doesn't cause ENFILE).

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/cachefiles/Makefile    |    1 +
 fs/cachefiles/interface.c |    3 ++
 fs/cachefiles/internal.h  |   14 +++++++
 fs/cachefiles/io.c        |   88 +++++++++++++++++++++++++++++++++++++++++++++
 fs/cachefiles/namei.c     |    3 ++
 5 files changed, 109 insertions(+)
 create mode 100644 fs/cachefiles/io.c

diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
index 3455d3646547..d894d317d6e7 100644
--- a/fs/cachefiles/Makefile
+++ b/fs/cachefiles/Makefile
@@ -7,6 +7,7 @@ cachefiles-y := \
 	bind.o \
 	daemon.o \
 	interface.o \
+	io.o \
 	key.o \
 	main.o \
 	namei.o \
diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index 47596b58c2da..747211363ae0 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -469,4 +469,7 @@ const struct fscache_cache_ops cachefiles_cache_ops = {
 	.put_object		= cachefiles_put_object,
 	.get_object_usage	= cachefiles_get_object_usage,
 	.sync_cache		= cachefiles_sync_cache,
+	.shape_extent		= cachefiles_shape_extent,
+	.read			= cachefiles_read,
+	.write			= cachefiles_write,
 };
diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index 16d15291a629..9f9e33893b4f 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -115,6 +115,20 @@ extern const struct fscache_cache_ops cachefiles_cache_ops;
 extern struct fscache_object *cachefiles_grab_object(struct fscache_object *_object,
 						     enum fscache_obj_ref_trace why);
 
+/*
+ * io.c
+ */
+extern unsigned int cachefiles_shape_extent(struct fscache_object *object,
+					    struct fscache_extent *extent,
+					    loff_t i_size, bool for_write);
+extern int cachefiles_read(struct fscache_object *object,
+			   struct fscache_io_request *req,
+			   struct iov_iter *iter);
+extern int cachefiles_write(struct fscache_object *object,
+			    struct fscache_io_request *req,
+			    struct iov_iter *iter);
+extern bool cachefiles_open_object(struct cachefiles_object *obj);
+
 /*
  * key.c
  */
diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
new file mode 100644
index 000000000000..d472e9d9173c
--- /dev/null
+++ b/fs/cachefiles/io.c
@@ -0,0 +1,88 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Data I/O routines
+ *
+ * Copyright (C) 2020 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@xxxxxxxxxx)
+ */
+
+#include <linux/mount.h>
+#include <linux/slab.h>
+#include <linux/file.h>
+#include <linux/uio.h>
+#include <linux/xattr.h>
+#include "internal.h"
+
+/*
+ * Determine the size of a data extent in a cache object.  This must be written
+ * as a whole unit, but can be read piecemeal.
+ */
+unsigned int cachefiles_shape_extent(struct fscache_object *object,
+				     struct fscache_extent *extent,
+				     loff_t i_size, bool for_write)
+{
+	return 0;
+}
+
+/*
+ * Initiate a read from the cache.
+ */
+int cachefiles_read(struct fscache_object *object,
+		    struct fscache_io_request *req,
+		    struct iov_iter *iter)
+{
+	req->error = -ENODATA;
+	if (req->io_done)
+		req->io_done(req);
+	return -ENODATA;
+}
+
+/*
+ * Initiate a write to the cache.
+ */
+int cachefiles_write(struct fscache_object *object,
+		     struct fscache_io_request *req,
+		     struct iov_iter *iter)
+{
+	req->error = -ENOBUFS;
+	if (req->io_done)
+		req->io_done(req);
+	return -ENOBUFS;
+}
+
+/*
+ * Open a cache object.
+ */
+bool cachefiles_open_object(struct cachefiles_object *object)
+{
+	struct cachefiles_cache *cache =
+		container_of(object->fscache.cache, struct cachefiles_cache, cache);
+	struct file *file;
+	struct path path;
+
+	path.mnt = cache->mnt;
+	path.dentry = object->backer;
+
+	file = open_with_fake_path(&path,
+				   O_RDWR | O_LARGEFILE | O_DIRECT,
+				   d_backing_inode(object->backer),
+				   cache->cache_cred);
+	if (IS_ERR(file))
+		goto error;
+
+	if (!S_ISREG(file_inode(file)->i_mode))
+		goto error_file;
+
+	if (unlikely(!file->f_op->read_iter) ||
+	    unlikely(!file->f_op->write_iter)) {
+		pr_notice("Cache does not support read_iter and write_iter\n");
+		goto error_file;
+	}
+
+	object->backing_file = file;
+	return true;
+
+error_file:
+	fput(file);
+error:
+	return false;
+}
diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index ad7edd41ce27..c33b5ee65ac1 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -492,6 +492,9 @@ bool cachefiles_walk_to_object(struct cachefiles_object *parent,
 		} else {
 			BUG(); // TODO: open file in data-class subdir
 		}
+
+		if (!cachefiles_open_object(object))
+			goto check_error;
 	}
 
 	if (object->new)