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Subject[PATCH 13/45] FS-Cache: Add cache tag handling [ver #41]
Date
Implement two features of FS-Cache:

(1) The ability to request and release cache tags - names by which a cache may
be known to a netfs, and thus selected for use.

(2) An internal function by which a cache is selected by consulting the netfs,
if the netfs wishes to be consulted.

Signed-off-by: David Howells <dhowells@redhat.com>
---
fs/fscache/Makefile | 1
fs/fscache/fsc-cache.c | 166 +++++++++++++++++++++++++++++++++++++++++++++
fs/fscache/fsc-internal.h | 20 +++++
include/linux/fscache.h | 9 ++
4 files changed, 195 insertions(+), 1 deletions(-)
create mode 100644 fs/fscache/fsc-cache.c

diff --git a/fs/fscache/Makefile b/fs/fscache/Makefile
index 8fb7bcc..85d1dd4 100644
--- a/fs/fscache/Makefile
+++ b/fs/fscache/Makefile
@@ -3,6 +3,7 @@
#

fscache-y := \
+ fsc-cache.o \
fsc-fsdef.o \
fsc-main.o \
fsc-proc.o \
diff --git a/fs/fscache/fsc-cache.c b/fs/fscache/fsc-cache.c
new file mode 100644
index 0000000..4b28536
--- /dev/null
+++ b/fs/fscache/fsc-cache.c
@@ -0,0 +1,166 @@
+/* FS-Cache cache handling
+ *
+ * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+
+#define FSCACHE_DEBUG_LEVEL CACHE
+#include <linux/module.h>
+#include <linux/slab.h>
+#include "fsc-internal.h"
+
+LIST_HEAD(fscache_cache_list);
+DECLARE_RWSEM(fscache_addremove_sem);
+
+static LIST_HEAD(fscache_cache_tag_list);
+
+/*
+ * look up a cache tag
+ */
+struct fscache_cache_tag *__fscache_lookup_cache_tag(const char *name)
+{
+ struct fscache_cache_tag *tag, *xtag;
+
+ /* firstly check for the existence of the tag under read lock */
+ down_read(&fscache_addremove_sem);
+
+ list_for_each_entry(tag, &fscache_cache_tag_list, link) {
+ if (strcmp(tag->name, name) == 0) {
+ atomic_inc(&tag->usage);
+ up_read(&fscache_addremove_sem);
+ return tag;
+ }
+ }
+
+ up_read(&fscache_addremove_sem);
+
+ /* the tag does not exist - create a candidate */
+ xtag = kzalloc(sizeof(*xtag) + strlen(name) + 1, GFP_KERNEL);
+ if (!xtag)
+ /* return a dummy tag if out of memory */
+ return ERR_PTR(-ENOMEM);
+
+ atomic_set(&xtag->usage, 1);
+ strcpy(xtag->name, name);
+
+ /* write lock, search again and add if still not present */
+ down_write(&fscache_addremove_sem);
+
+ list_for_each_entry(tag, &fscache_cache_tag_list, link) {
+ if (strcmp(tag->name, name) == 0) {
+ atomic_inc(&tag->usage);
+ up_write(&fscache_addremove_sem);
+ kfree(xtag);
+ return tag;
+ }
+ }
+
+ list_add_tail(&xtag->link, &fscache_cache_tag_list);
+ up_write(&fscache_addremove_sem);
+ return xtag;
+}
+
+/*
+ * release a reference to a cache tag
+ */
+void __fscache_release_cache_tag(struct fscache_cache_tag *tag)
+{
+ if (tag != ERR_PTR(-ENOMEM)) {
+ down_write(&fscache_addremove_sem);
+
+ if (atomic_dec_and_test(&tag->usage))
+ list_del_init(&tag->link);
+ else
+ tag = NULL;
+
+ up_write(&fscache_addremove_sem);
+
+ kfree(tag);
+ }
+}
+
+/*
+ * select a cache in which to store an object
+ * - the cache addremove semaphore must be at least read-locked by the caller
+ * - the object will never be an index
+ */
+struct fscache_cache *fscache_select_cache_for_object(
+ struct fscache_cookie *cookie)
+{
+ struct fscache_cache_tag *tag;
+ struct fscache_object *object;
+ struct fscache_cache *cache;
+
+ _enter("");
+
+ if (list_empty(&fscache_cache_list)) {
+ _leave(" = NULL [no cache]");
+ return NULL;
+ }
+
+ /* we check the parent to determine the cache to use */
+ spin_lock(&cookie->lock);
+
+ /* the first in the parent's backing list should be the preferred
+ * cache */
+ if (!hlist_empty(&cookie->backing_objects)) {
+ object = hlist_entry(cookie->backing_objects.first,
+ struct fscache_object, cookie_link);
+
+ cache = object->cache;
+ if (object->state >= FSCACHE_OBJECT_DYING ||
+ test_bit(FSCACHE_IOERROR, &cache->flags))
+ cache = NULL;
+
+ spin_unlock(&cookie->lock);
+ _leave(" = %p [parent]", cache);
+ return cache;
+ }
+
+ /* the parent is unbacked */
+ if (cookie->def->type != FSCACHE_COOKIE_TYPE_INDEX) {
+ /* cookie not an index and is unbacked */
+ spin_unlock(&cookie->lock);
+ _leave(" = NULL [cookie ub,ni]");
+ return NULL;
+ }
+
+ spin_unlock(&cookie->lock);
+
+ if (!cookie->def->select_cache)
+ goto no_preference;
+
+ /* ask the netfs for its preference */
+ tag = cookie->def->select_cache(cookie->parent->netfs_data,
+ cookie->netfs_data);
+ if (!tag)
+ goto no_preference;
+
+ if (tag == ERR_PTR(-ENOMEM)) {
+ _leave(" = NULL [nomem tag]");
+ return NULL;
+ }
+
+ if (!tag->cache) {
+ _leave(" = NULL [unbacked tag]");
+ return NULL;
+ }
+
+ if (test_bit(FSCACHE_IOERROR, &tag->cache->flags))
+ return NULL;
+
+ _leave(" = %p [specific]", tag->cache);
+ return tag->cache;
+
+no_preference:
+ /* netfs has no preference - just select first cache */
+ cache = list_entry(fscache_cache_list.next,
+ struct fscache_cache, link);
+ _leave(" = %p [first]", cache);
+ return cache;
+}
diff --git a/fs/fscache/fsc-internal.h b/fs/fscache/fsc-internal.h
index 5ff611a..ffd2c88 100644
--- a/fs/fscache/fsc-internal.h
+++ b/fs/fscache/fsc-internal.h
@@ -28,6 +28,15 @@
#define FSCACHE_MAX_THREADS 32

/*
+ * fsc-cache.c
+ */
+extern struct list_head fscache_cache_list;
+extern struct rw_semaphore fscache_addremove_sem;
+
+extern struct fscache_cache *fscache_select_cache_for_object(
+ struct fscache_cookie *);
+
+/*
* fsc-fsdef.c
*/
extern struct fscache_cookie fscache_fsdef_index;
@@ -158,6 +167,17 @@ extern void fscache_proc_cleanup(void);
#define fscache_proc_cleanup() do {} while (0)
#endif

+/*
+ * raise an event on an object
+ * - if the event is not masked for that object, then the object is
+ * queued for attention by the thread pool.
+ */
+static inline void fscache_raise_event(struct fscache_object *object,
+ unsigned event)
+{
+ BUG(); // TODO
+}
+
/*****************************************************************************/
/*
* debug tracing
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index ea0fd0f..0b66726 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -187,6 +187,8 @@ struct fscache_netfs {
* - these are undefined symbols when FS-Cache is not configured and the
* optimiser takes care of not using them
*/
+extern struct fscache_cache_tag *__fscache_lookup_cache_tag(const char *);
+extern void __fscache_release_cache_tag(struct fscache_cache_tag *);

/**
* fscache_register_netfs - Register a filesystem as desiring caching services
@@ -232,7 +234,10 @@ void fscache_unregister_netfs(struct fscache_netfs *netfs)
static inline
struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name)
{
- return NULL;
+ if (fscache_available())
+ return __fscache_lookup_cache_tag(name);
+ else
+ return NULL;
}

/**
@@ -247,6 +252,8 @@ struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name)
static inline
void fscache_release_cache_tag(struct fscache_cache_tag *tag)
{
+ if (fscache_available())
+ __fscache_release_cache_tag(tag);
}

/**


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