FS-Cache: Add cache tag handling

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>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>

3 files changed, 187 insertions, 0 deletions

diff --git a/fs/fscache/Makefile b/fs/fscache/Makefile
index bc1f3b9d811a..556708bb9796 100644
--- a/fs/fscache/Makefile
+++ b/fs/fscache/Makefile
@@ -3,6 +3,7 @@
 #
 
 fscache-y := \
+        cache.o \
         fsdef.o \
         main.o
 
diff --git a/fs/fscache/cache.c b/fs/fscache/cache.c
new file mode 100644
index 000000000000..1a28df36dd93
--- /dev/null
+++ b/fs/fscache/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 "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/internal.h b/fs/fscache/internal.h
index 4113af8d1660..0a2069afa417 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/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;
@@ -168,6 +177,17 @@ extern const struct file_operations fscache_stats_fops;
 #define fscache_stat(stat) 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