[PATCH v2 49/67] cachefiles: Implement begin and end I/O operation

[David Howells <dhowells@redhat.com>]

Implement the methods for beginning and ending an I/O operation.

When called to begin an I/O operation, we are guaranteed that the cookie
has reached a certain stage (we're called by fscache after it has done a
suitable wait).

If a file is available, we paste a ref over into the cache resources for
the I/O routines to use.  This means that the object can be invalidated
whilst the I/O is ongoing without the need to synchronise as the file
pointer in the object is replaced, but the file pointer in the cache
resources is unaffected.

Ending the operation just requires ditching any refs we have and dropping
the access guarantee that fscache got for us on the cookie.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819645033.215744.2199344081658268312.stgit@warthog.procyon.org.uk/ # v1
---

 fs/cachefiles/Makefile         |    1 +
 fs/cachefiles/interface.c      |    1 +
 fs/cachefiles/internal.h       |   18 +++++++++++++
 fs/cachefiles/io.c             |   57 ++++++++++++++++++++++++++++++++++++++++
 include/trace/events/fscache.h |    2 +
 5 files changed, 79 insertions(+)
 create mode 100644 fs/cachefiles/io.c

diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
index cb7a6bcf51eb..16d811f1a2fa 100644
--- a/fs/cachefiles/Makefile
+++ b/fs/cachefiles/Makefile
@@ -7,6 +7,7 @@ cachefiles-y := \
 	cache.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 e47c52c34071..ad9d311413ff 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -362,5 +362,6 @@ const struct fscache_cache_ops cachefiles_cache_ops = {
 	.lookup_cookie		= cachefiles_lookup_cookie,
 	.withdraw_cookie	= cachefiles_withdraw_cookie,
 	.invalidate_cookie	= cachefiles_invalidate_cookie,
+	.begin_operation	= cachefiles_begin_operation,
 	.prepare_to_write	= cachefiles_prepare_to_write,
 };
diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index c10c04593f5a..77c899b3eaa5 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -105,6 +105,18 @@ struct cachefiles_cache {
 
 #include <trace/events/cachefiles.h>
 
+static inline
+struct file *cachefiles_cres_file(struct netfs_cache_resources *cres)
+{
+	return cres->cache_priv2;
+}
+
+static inline
+struct cachefiles_object *cachefiles_cres_object(struct netfs_cache_resources *cres)
+{
+	return fscache_cres_cookie(cres)->cache_priv;
+}
+
 /*
  * note change of state for daemon
  */
@@ -177,6 +189,12 @@ extern struct cachefiles_object *cachefiles_grab_object(struct cachefiles_object
 extern void cachefiles_put_object(struct cachefiles_object *object,
 				  enum cachefiles_obj_ref_trace why);
 
+/*
+ * io.c
+ */
+extern bool cachefiles_begin_operation(struct netfs_cache_resources *cres,
+				       enum fscache_want_state want_state);
+
 /*
  * key.c
  */
diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
new file mode 100644
index 000000000000..adeb9a42fd7b
--- /dev/null
+++ b/fs/cachefiles/io.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* kiocb-using read/write
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/mount.h>
+#include <linux/slab.h>
+#include <linux/file.h>
+#include <linux/uio.h>
+#include <linux/falloc.h>
+#include <linux/sched/mm.h>
+#include <trace/events/fscache.h>
+#include "internal.h"
+
+/*
+ * Clean up an operation.
+ */
+static void cachefiles_end_operation(struct netfs_cache_resources *cres)
+{
+	struct file *file = cachefiles_cres_file(cres);
+
+	if (file)
+		fput(file);
+	fscache_end_cookie_access(fscache_cres_cookie(cres), fscache_access_io_end);
+}
+
+static const struct netfs_cache_ops cachefiles_netfs_cache_ops = {
+	.end_operation		= cachefiles_end_operation,
+};
+
+/*
+ * Open the cache file when beginning a cache operation.
+ */
+bool cachefiles_begin_operation(struct netfs_cache_resources *cres,
+				enum fscache_want_state want_state)
+{
+	struct cachefiles_object *object = cachefiles_cres_object(cres);
+
+	if (!cachefiles_cres_file(cres)) {
+		cres->ops = &cachefiles_netfs_cache_ops;
+		if (object->file) {
+			spin_lock(&object->lock);
+			if (!cres->cache_priv2 && object->file)
+				cres->cache_priv2 = get_file(object->file);
+			spin_unlock(&object->lock);
+		}
+	}
+
+	if (!cachefiles_cres_file(cres) && want_state != FSCACHE_WANT_PARAMS) {
+		pr_err("failed to get cres->file\n");
+		return false;
+	}
+
+	return true;
+}
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index d9d830296ec3..1594aefadeac 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -78,6 +78,7 @@ enum fscache_access_trace {
 	fscache_access_cache_unpin,
 	fscache_access_invalidate_cookie,
 	fscache_access_invalidate_cookie_end,
+	fscache_access_io_end,
 	fscache_access_io_not_live,
 	fscache_access_io_read,
 	fscache_access_io_resize,
@@ -152,6 +153,7 @@ enum fscache_access_trace {
 	EM(fscache_access_cache_unpin,		"UNPIN cache  ")	\
 	EM(fscache_access_invalidate_cookie,	"BEGIN inval  ")	\
 	EM(fscache_access_invalidate_cookie_end,"END   inval  ")	\
+	EM(fscache_access_io_end,		"END   io     ")	\
 	EM(fscache_access_io_not_live,		"END   io_notl")	\
 	EM(fscache_access_io_read,		"BEGIN io_read")	\
 	EM(fscache_access_io_resize,		"BEGIN io_resz")	\