fscache: Implement cookie user counting and resource pinning

Provide a pair of functions to count the number of users of a cookie (open
files, writeback, invalidation, resizing, reads, writes), to obtain and pin
resources for the cookie and to prevent culling for the whilst there are
users.

The first function marks a cookie as being in use:

	void fscache_use_cookie(struct fscache_cookie *cookie,
				bool will_modify);

The caller should indicate the cookie to use and whether or not the caller
is in a context that may modify the cookie (e.g. a file open O_RDWR).

If the cookie is not already resourced, fscache will ask the cache backend
in the background to do whatever it needs to look up, create or otherwise
obtain the resources necessary to access data.  This is pinned to the
cookie and may not be culled, though it may be withdrawn if the cache as a
whole is withdrawn.

The second function removes the in-use mark from a cookie and, optionally,
updates the coherency data:

	void fscache_unuse_cookie(struct fscache_cookie *cookie,
				  const void *aux_data,
				  const loff_t *object_size);

If non-NULL, the aux_data buffer and/or the object_size will be saved into
the cookie and will be set on the backing store when the object is
committed.

If this removes the last usage on a cookie, the cookie is placed onto an
LRU list from which it will be removed and closed after a couple of seconds
if it doesn't get reused.  This prevents resource overload in the cache -
in particular it prevents it from holding too many files open.

Changes
=======
ver #2:
 - Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a
   potential race if the cookie gets reused before it completes the
   unusement.
 - Added missing transition to LRU_DISCARDING state.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4

1 files changed, 81 insertions, 1 deletions

diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 4450d17c11e8..e6c321e5bf73 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -22,12 +22,14 @@
 #define fscache_available() (1)
 #define fscache_volume_valid(volume) (volume)
 #define fscache_cookie_valid(cookie) (cookie)
-#define fscache_cookie_enabled(cookie) (cookie)
+#define fscache_resources_valid(cres) ((cres)->cache_priv)
+#define fscache_cookie_enabled(cookie) (cookie && !test_bit(FSCACHE_COOKIE_DISABLED, &cookie->flags))
 #else
 #define __fscache_available (0)
 #define fscache_available() (0)
 #define fscache_volume_valid(volume) (0)
 #define fscache_cookie_valid(cookie) (0)
+#define fscache_resources_valid(cres) (false)
 #define fscache_cookie_enabled(cookie) (0)
 #endif
 
@@ -46,6 +48,7 @@ enum fscache_cookie_state {
 	FSCACHE_COOKIE_STATE_CREATING,		/* The cache object is being created */
 	FSCACHE_COOKIE_STATE_ACTIVE,		/* The cache is active, readable and writable */
 	FSCACHE_COOKIE_STATE_FAILED,		/* The cache failed, withdraw to clear */
+	FSCACHE_COOKIE_STATE_LRU_DISCARDING,	/* The cookie is being discarded by the LRU */
 	FSCACHE_COOKIE_STATE_WITHDRAWING,	/* The cookie is being withdrawn */
 	FSCACHE_COOKIE_STATE_RELINQUISHING,	/* The cookie is being relinquished */
 	FSCACHE_COOKIE_STATE_DROPPED,		/* The cookie has been dropped */
@@ -147,6 +150,8 @@ extern struct fscache_cookie *__fscache_acquire_cookie(
 	const void *, size_t,
 	const void *, size_t,
 	loff_t);
+extern void __fscache_use_cookie(struct fscache_cookie *, bool);
+extern void __fscache_unuse_cookie(struct fscache_cookie *, const void *, const loff_t *);
 extern void __fscache_relinquish_cookie(struct fscache_cookie *, bool);
 
 /**
@@ -229,6 +234,39 @@ struct fscache_cookie *fscache_acquire_cookie(struct fscache_volume *volume,
 }
 
 /**
+ * fscache_use_cookie - Request usage of cookie attached to an object
+ * @object: Object description
+ * @will_modify: If cache is expected to be modified locally
+ *
+ * Request usage of the cookie attached to an object.  The caller should tell
+ * the cache if the object's contents are about to be modified locally and then
+ * the cache can apply the policy that has been set to handle this case.
+ */
+static inline void fscache_use_cookie(struct fscache_cookie *cookie,
+				      bool will_modify)
+{
+	if (fscache_cookie_valid(cookie))
+		__fscache_use_cookie(cookie, will_modify);
+}
+
+/**
+ * fscache_unuse_cookie - Cease usage of cookie attached to an object
+ * @object: Object description
+ * @aux_data: Updated auxiliary data (or NULL)
+ * @object_size: Revised size of the object (or NULL)
+ *
+ * Cease usage of the cookie attached to an object.  When the users count
+ * reaches zero then the cookie relinquishment will be permitted to proceed.
+ */
+static inline void fscache_unuse_cookie(struct fscache_cookie *cookie,
+					const void *aux_data,
+					const loff_t *object_size)
+{
+	if (fscache_cookie_valid(cookie))
+		__fscache_unuse_cookie(cookie, aux_data, object_size);
+}
+
+/**
  * fscache_relinquish_cookie - Return the cookie to the cache, maybe discarding
  * it
  * @cookie: The cookie being returned
@@ -247,4 +285,46 @@ void fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire)
 		__fscache_relinquish_cookie(cookie, retire);
 }
 
+/*
+ * Find the auxiliary data on a cookie.
+ */
+static inline void *fscache_get_aux(struct fscache_cookie *cookie)
+{
+	if (cookie->aux_len <= sizeof(cookie->inline_aux))
+		return cookie->inline_aux;
+	else
+		return cookie->aux;
+}
+
+/*
+ * Update the auxiliary data on a cookie.
+ */
+static inline
+void fscache_update_aux(struct fscache_cookie *cookie,
+			const void *aux_data, const loff_t *object_size)
+{
+	void *p = fscache_get_aux(cookie);
+
+	if (aux_data && p)
+		memcpy(p, aux_data, cookie->aux_len);
+	if (object_size)
+		cookie->object_size = *object_size;
+}
+
+#ifdef CONFIG_FSCACHE_STATS
+extern atomic_t fscache_n_updates;
+#endif
+
+static inline
+void __fscache_update_cookie(struct fscache_cookie *cookie, const void *aux_data,
+			     const loff_t *object_size)
+{
+#ifdef CONFIG_FSCACHE_STATS
+	atomic_inc(&fscache_n_updates);
+#endif
+	fscache_update_aux(cookie, aux_data, object_size);
+	smp_wmb();
+	set_bit(FSCACHE_COOKIE_NEEDS_UPDATE, &cookie->flags);
+}
+
 #endif /* _LINUX_FSCACHE_H */