FS-Cache: Provide the ability to enable/disable cookies

Provide the ability to enable and disable fscache cookies.  A disabled cookie
will reject or ignore further requests to:

	Acquire a child cookie
	Invalidate and update backing objects
	Check the consistency of a backing object
	Allocate storage for backing page
	Read backing pages
	Write to backing pages

but still allows:

	Checks/waits on the completion of already in-progress objects
	Uncaching of pages
	Relinquishment of cookies

Two new operations are provided:

 (1) Disable a cookie:

	void fscache_disable_cookie(struct fscache_cookie *cookie,
				    bool invalidate);

     If the cookie is not already disabled, this locks the cookie against other
     dis/enablement ops, marks the cookie as being disabled, discards or
     invalidates any backing objects and waits for cessation of activity on any
     associated object.

     This is a wrapper around a chunk split out of fscache_relinquish_cookie(),
     but it reinitialises the cookie such that it can be reenabled.

     All possible failures are handled internally.  The caller should consider
     calling fscache_uncache_all_inode_pages() afterwards to make sure all page
     markings are cleared up.

 (2) Enable a cookie:

	void fscache_enable_cookie(struct fscache_cookie *cookie,
				   bool (*can_enable)(void *data),
				   void *data)

     If the cookie is not already enabled, this locks the cookie against other
     dis/enablement ops, invokes can_enable() and, if the cookie is not an
     index cookie, will begin the procedure of acquiring backing objects.

     The optional can_enable() function is passed the data argument and returns
     a ruling as to whether or not enablement should actually be permitted to
     begin.

     All possible failures are handled internally.  The cookie will only be
     marked as enabled if provisional backing objects are allocated.

A later patch will introduce these to NFS.  Cookie enablement during nfs_open()
is then contingent on i_writecount <= 0.  can_enable() checks for a race
between open(O_RDONLY) and open(O_WRONLY/O_RDWR).  This simplifies NFS's cookie
handling and allows us to get rid of open(O_RDONLY) accidentally introducing
caching to an inode that's open for writing already.

One operation has its API modified:

 (3) Acquire a cookie.

	struct fscache_cookie *fscache_acquire_cookie(
		struct fscache_cookie *parent,
		const struct fscache_cookie_def *def,
		void *netfs_data,
		bool enable);

     This now has an additional argument that indicates whether the requested
     cookie should be enabled by default.  It doesn't need the can_enable()
     function because the caller must prevent multiple calls for the same netfs
     object and it doesn't need to take the enablement lock because no one else
     can get at the cookie before this returns.

Signed-off-by: David Howells <dhowells@redhat.com

1 files changed, 99 insertions, 14 deletions

diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 19b46458e4e8..115bb81912cc 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -167,6 +167,42 @@ struct fscache_netfs {
 };
 
 /*
+ * data file or index object cookie
+ * - a file will only appear in one cache
+ * - a request to cache a file may or may not be honoured, subject to
+ *   constraints such as disk space
+ * - indices are created on disk just-in-time
+ */
+struct fscache_cookie {
+	atomic_t			usage;		/* number of users of this cookie */
+	atomic_t			n_children;	/* number of children of this cookie */
+	atomic_t			n_active;	/* number of active users of netfs ptrs */
+	spinlock_t			lock;
+	spinlock_t			stores_lock;	/* lock on page store tree */
+	struct hlist_head		backing_objects; /* object(s) backing this file/index */
+	const struct fscache_cookie_def	*def;		/* definition */
+	struct fscache_cookie		*parent;	/* parent of this entry */
+	void				*netfs_data;	/* back pointer to netfs */
+	struct radix_tree_root		stores;		/* pages to be stored on this cookie */
+#define FSCACHE_COOKIE_PENDING_TAG	0		/* pages tag: pending write to cache */
+#define FSCACHE_COOKIE_STORING_TAG	1		/* pages tag: writing to cache */
+
+	unsigned long			flags;
+#define FSCACHE_COOKIE_LOOKING_UP	0	/* T if non-index cookie being looked up still */
+#define FSCACHE_COOKIE_NO_DATA_YET	1	/* T if new object with no cached data yet */
+#define FSCACHE_COOKIE_UNAVAILABLE	2	/* T if cookie is unavailable (error, etc) */
+#define FSCACHE_COOKIE_INVALIDATING	3	/* T if cookie is being invalidated */
+#define FSCACHE_COOKIE_RELINQUISHED	4	/* T if cookie has been relinquished */
+#define FSCACHE_COOKIE_ENABLED		5	/* T if cookie is enabled */
+#define FSCACHE_COOKIE_ENABLEMENT_LOCK	6	/* T if cookie is being en/disabled */
+};
+
+static inline bool fscache_cookie_enabled(struct fscache_cookie *cookie)
+{
+	return test_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+}
+
+/*
  * slow-path functions for when there is actually caching available, and the
  * netfs does actually have a valid token
  * - these are not to be called directly
@@ -181,8 +217,8 @@ extern void __fscache_release_cache_tag(struct fscache_cache_tag *);
 extern struct fscache_cookie *__fscache_acquire_cookie(
 	struct fscache_cookie *,
 	const struct fscache_cookie_def *,
-	void *);
-extern void __fscache_relinquish_cookie(struct fscache_cookie *, int);
+	void *, bool);
+extern void __fscache_relinquish_cookie(struct fscache_cookie *, bool);
 extern int __fscache_check_consistency(struct fscache_cookie *);
 extern void __fscache_update_cookie(struct fscache_cookie *);
 extern int __fscache_attr_changed(struct fscache_cookie *);
@@ -211,6 +247,9 @@ extern void __fscache_uncache_all_inode_pages(struct fscache_cookie *,
 					      struct inode *);
 extern void __fscache_readpages_cancel(struct fscache_cookie *cookie,
 				       struct list_head *pages);
+extern void __fscache_disable_cookie(struct fscache_cookie *, bool);
+extern void __fscache_enable_cookie(struct fscache_cookie *,
+				    bool (*)(void *), void *);
 
 /**
  * fscache_register_netfs - Register a filesystem as desiring caching services
@@ -289,6 +328,7 @@ void fscache_release_cache_tag(struct fscache_cache_tag *tag)
  * @def: A description of the cache object, including callback operations
  * @netfs_data: An arbitrary piece of data to be kept in the cookie to
  * represent the cache object to the netfs
+ * @enable: Whether or not to enable a data cookie immediately
  *
  * This function is used to inform FS-Cache about part of an index hierarchy
  * that can be used to locate files.  This is done by requesting a cookie for
@@ -301,10 +341,12 @@ static inline
 struct fscache_cookie *fscache_acquire_cookie(
 	struct fscache_cookie *parent,
 	const struct fscache_cookie_def *def,
-	void *netfs_data)
+	void *netfs_data,
+	bool enable)
 {
-	if (fscache_cookie_valid(parent))
-		return __fscache_acquire_cookie(parent, def, netfs_data);
+	if (fscache_cookie_valid(parent) && fscache_cookie_enabled(parent))
+		return __fscache_acquire_cookie(parent, def, netfs_data,
+						enable);
 	else
 		return NULL;
 }
@@ -322,7 +364,7 @@ struct fscache_cookie *fscache_acquire_cookie(
  * description.
  */
 static inline
-void fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
+void fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire)
 {
 	if (fscache_cookie_valid(cookie))
 		__fscache_relinquish_cookie(cookie, retire);
@@ -341,7 +383,7 @@ void fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
 static inline
 int fscache_check_consistency(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_check_consistency(cookie);
 	else
 		return 0;
@@ -360,7 +402,7 @@ int fscache_check_consistency(struct fscache_cookie *cookie)
 static inline
 void fscache_update_cookie(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		__fscache_update_cookie(cookie);
 }
 
@@ -407,7 +449,7 @@ void fscache_unpin_cookie(struct fscache_cookie *cookie)
 static inline
 int fscache_attr_changed(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_attr_changed(cookie);
 	else
 		return -ENOBUFS;
@@ -429,7 +471,7 @@ int fscache_attr_changed(struct fscache_cookie *cookie)
 static inline
 void fscache_invalidate(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		__fscache_invalidate(cookie);
 }
 
@@ -503,7 +545,7 @@ int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
 			       void *context,
 			       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_read_or_alloc_page(cookie, page, end_io_func,
 						    context, gfp);
 	else
@@ -554,7 +596,7 @@ int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
 				void *context,
 				gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_read_or_alloc_pages(cookie, mapping, pages,
 						     nr_pages, end_io_func,
 						     context, gfp);
@@ -585,7 +627,7 @@ int fscache_alloc_page(struct fscache_cookie *cookie,
 		       struct page *page,
 		       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_alloc_page(cookie, page, gfp);
 	else
 		return -ENOBUFS;
@@ -634,7 +676,7 @@ int fscache_write_page(struct fscache_cookie *cookie,
 		       struct page *page,
 		       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_write_page(cookie, page, gfp);
 	else
 		return -ENOBUFS;
@@ -744,4 +786,47 @@ void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
 		__fscache_uncache_all_inode_pages(cookie, inode);
 }
 
+/**
+ * fscache_disable_cookie - Disable a cookie
+ * @cookie: The cookie representing the cache object
+ * @invalidate: Invalidate the backing object
+ *
+ * Disable a cookie from accepting further alloc, read, write, invalidate,
+ * update or acquire operations.  Outstanding operations can still be waited
+ * upon and pages can still be uncached and the cookie relinquished.
+ *
+ * This will not return until all outstanding operations have completed.
+ *
+ * If @invalidate is set, then the backing object will be invalidated and
+ * detached, otherwise it will just be detached.
+ */
+static inline
+void fscache_disable_cookie(struct fscache_cookie *cookie, bool invalidate)
+{
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
+		__fscache_disable_cookie(cookie, invalidate);
+}
+
+/**
+ * fscache_enable_cookie - Reenable a cookie
+ * @cookie: The cookie representing the cache object
+ * @can_enable: A function to permit enablement once lock is held
+ * @data: Data for can_enable()
+ *
+ * Reenable a previously disabled cookie, allowing it to accept further alloc,
+ * read, write, invalidate, update or acquire operations.  An attempt will be
+ * made to immediately reattach the cookie to a backing object.
+ *
+ * The can_enable() function is called (if not NULL) once the enablement lock
+ * is held to rule on whether enablement is still permitted to go ahead.
+ */
+static inline
+void fscache_enable_cookie(struct fscache_cookie *cookie,
+			   bool (*can_enable)(void *data),
+			   void *data)
+{
+	if (fscache_cookie_valid(cookie) && !fscache_cookie_enabled(cookie))
+		__fscache_enable_cookie(cookie, can_enable, data);
+}
+
 #endif /* _LINUX_FSCACHE_H */