fscache: Provide and use cache methods to lookup/create/free a volume

Add cache methods to lookup, create and remove a volume.

Looking up or creating the volume requires the cache pinning for access;
freeing the volume requires the volume pinning for access.  The
->acquire_volume() method is used to ask the cache backend to lookup and,
if necessary, create a volume; the ->free_volume() method is used to free
the resources for a volume.

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/163819597821.215744.5225318658134989949.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906898645.143852.8537799955945956818.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967099771.1823006.1455197910571061835.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021507345.640689.4073511598838843040.stgit@warthog.procyon.org.uk/ # v4

1 files changed, 86 insertions, 3 deletions

diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index 20497f0f10bb..e1a8e92a6adb 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -15,6 +15,8 @@ static struct hlist_bl_head fscache_volume_hash[1 << fscache_volume_hash_shift];
 static atomic_t fscache_volume_debug_id;
 static LIST_HEAD(fscache_volumes);
 
+static void fscache_create_volume_work(struct work_struct *work);
+
 struct fscache_volume *fscache_get_volume(struct fscache_volume *volume,
 					  enum fscache_volume_trace where)
 {
@@ -213,7 +215,7 @@ static struct fscache_volume *fscache_alloc_volume(const char *volume_key,
 
 	volume->cache = cache;
 	INIT_LIST_HEAD(&volume->proc_link);
-	INIT_WORK(&volume->work, NULL /* PLACEHOLDER */);
+	INIT_WORK(&volume->work, fscache_create_volume_work);
 	refcount_set(&volume->ref, 1);
 	spin_lock_init(&volume->lock);
 
@@ -250,6 +252,58 @@ err_cache:
 }
 
 /*
+ * Create a volume's representation on disk.  Have a volume ref and a cache
+ * access we have to release.
+ */
+static void fscache_create_volume_work(struct work_struct *work)
+{
+	const struct fscache_cache_ops *ops;
+	struct fscache_volume *volume =
+		container_of(work, struct fscache_volume, work);
+
+	fscache_see_volume(volume, fscache_volume_see_create_work);
+
+	ops = volume->cache->ops;
+	if (ops->acquire_volume)
+		ops->acquire_volume(volume);
+	fscache_end_cache_access(volume->cache,
+				 fscache_access_acquire_volume_end);
+
+	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
+	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
+	fscache_put_volume(volume, fscache_volume_put_create_work);
+}
+
+/*
+ * Dispatch a worker thread to create a volume's representation on disk.
+ */
+void fscache_create_volume(struct fscache_volume *volume, bool wait)
+{
+	if (test_and_set_bit(FSCACHE_VOLUME_CREATING, &volume->flags))
+		goto maybe_wait;
+	if (volume->cache_priv)
+		goto no_wait; /* We raced */
+	if (!fscache_begin_cache_access(volume->cache,
+					fscache_access_acquire_volume))
+		goto no_wait;
+
+	fscache_get_volume(volume, fscache_volume_get_create_work);
+	if (!schedule_work(&volume->work))
+		fscache_put_volume(volume, fscache_volume_put_create_work);
+
+maybe_wait:
+	if (wait) {
+		fscache_see_volume(volume, fscache_volume_wait_create_work);
+		wait_on_bit(&volume->flags, FSCACHE_VOLUME_CREATING,
+			    TASK_UNINTERRUPTIBLE);
+	}
+	return;
+no_wait:
+	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
+	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
+}
+
+/*
  * Acquire a volume representation cookie and link it to a (proposed) cache.
  */
 struct fscache_volume *__fscache_acquire_volume(const char *volume_key,
@@ -269,7 +323,7 @@ struct fscache_volume *__fscache_acquire_volume(const char *volume_key,
 		return ERR_PTR(-EBUSY);
 	}
 
-	// PLACEHOLDER: Create the volume if we have a cache available
+	fscache_create_volume(volume, false);
 	return volume;
 }
 EXPORT_SYMBOL(__fscache_acquire_volume);
@@ -316,7 +370,12 @@ static void fscache_free_volume(struct fscache_volume *volume)
 	struct fscache_cache *cache = volume->cache;
 
 	if (volume->cache_priv) {
-		// PLACEHOLDER: Detach any attached cache
+		__fscache_begin_volume_access(volume, NULL,
+					      fscache_access_relinquish_volume);
+		if (volume->cache_priv)
+			cache->ops->free_volume(volume);
+		fscache_end_volume_access(volume, NULL,
+					  fscache_access_relinquish_volume_end);
 	}
 
 	down_write(&fscache_addremove_sem);
@@ -369,6 +428,30 @@ void __fscache_relinquish_volume(struct fscache_volume *volume,
 }
 EXPORT_SYMBOL(__fscache_relinquish_volume);
 
+/**
+ * fscache_withdraw_volume - Withdraw a volume from being cached
+ * @volume: Volume cookie
+ *
+ * Withdraw a cache volume from service, waiting for all accesses to complete
+ * before returning.
+ */
+void fscache_withdraw_volume(struct fscache_volume *volume)
+{
+	int n_accesses;
+
+	_debug("withdraw V=%x", volume->debug_id);
+
+	/* Allow wakeups on dec-to-0 */
+	n_accesses = atomic_dec_return(&volume->n_accesses);
+	trace_fscache_access_volume(volume->debug_id, 0,
+				    refcount_read(&volume->ref),
+				    n_accesses, fscache_access_cache_unpin);
+
+	wait_var_event(&volume->n_accesses,
+		       atomic_read(&volume->n_accesses) == 0);
+}
+EXPORT_SYMBOL(fscache_withdraw_volume);
+
 #ifdef CONFIG_PROC_FS
 /*
  * Generate a list of volumes in /proc/fs/fscache/volumes