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cookie and may not be culled, though it may be withdrawn if the cache as a
whole is withdrawn.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

fs/fscache/cookie.c

@@ -15,14 +15,21 @@
 
 struct kmem_cache *fscache_cookie_jar;
 
+static void fscache_cookie_lru_timed_out(struct timer_list *timer);
+static void fscache_cookie_lru_worker(struct work_struct *work);
 static void fscache_cookie_worker(struct work_struct *work);
 static void fscache_unhash_cookie(struct fscache_cookie *cookie);
 
 #define fscache_cookie_hash_shift 15
 static struct hlist_bl_head fscache_cookie_hash[1 << fscache_cookie_hash_shift];
 static LIST_HEAD(fscache_cookies);
 static DEFINE_RWLOCK(fscache_cookies_lock);
-static const char fscache_cookie_states[FSCACHE_COOKIE_STATE__NR] = "-LCAFWRD";
+static LIST_HEAD(fscache_cookie_lru);
+static DEFINE_SPINLOCK(fscache_cookie_lru_lock);
+DEFINE_TIMER(fscache_cookie_lru_timer, fscache_cookie_lru_timed_out);
+static DECLARE_WORK(fscache_cookie_lru_work, fscache_cookie_lru_worker);
+static const char fscache_cookie_states[FSCACHE_COOKIE_STATE__NR] = "-LCAFUWRD";
+unsigned int fscache_lru_cookie_timeout = 10 * HZ;
 
 void fscache_print_cookie(struct fscache_cookie *cookie, char prefix)
 {
@@ -47,6 +54,14 @@ void fscache_print_cookie(struct fscache_cookie *cookie, char prefix)
 
 static void fscache_free_cookie(struct fscache_cookie *cookie)
 {
+	if (WARN_ON_ONCE(!list_empty(&cookie->commit_link))) {
+		spin_lock(&fscache_cookie_lru_lock);
+		list_del_init(&cookie->commit_link);
+		spin_unlock(&fscache_cookie_lru_lock);
+		fscache_stat_d(&fscache_n_cookies_lru);
+		fscache_stat(&fscache_n_cookies_lru_removed);
+	}
+
 	if (WARN_ON_ONCE(test_bit(FSCACHE_COOKIE_IS_HASHED, &cookie->flags))) {
 		fscache_print_cookie(cookie, 'F');
 		return;
@@ -498,6 +513,126 @@ static void fscache_perform_lookup(struct fscache_cookie *cookie)
 	fscache_end_volume_access(cookie->volume, cookie, trace);
 }
 
+/*
+ * Begin the process of looking up a cookie.  We offload the actual process to
+ * a worker thread.
+ */
+static bool fscache_begin_lookup(struct fscache_cookie *cookie, bool will_modify)
+{
+	if (will_modify) {
+		set_bit(FSCACHE_COOKIE_LOCAL_WRITE, &cookie->flags);
+		set_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
+	}
+	if (!fscache_begin_volume_access(cookie->volume, cookie,
+					 fscache_access_lookup_cookie))
+		return false;
+
+	__fscache_begin_cookie_access(cookie, fscache_access_lookup_cookie);
+	__fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_LOOKING_UP);
+	set_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags);
+	set_bit(FSCACHE_COOKIE_HAS_BEEN_CACHED, &cookie->flags);
+	return true;
+}
+
+/*
+ * Start using the cookie for I/O.  This prevents the backing object from being
+ * reaped by VM pressure.
+ */
+void __fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify)
+{
+	enum fscache_cookie_state state;
+	bool queue = false;
+
+	_enter("c=%08x", cookie->debug_id);
+
+	if (WARN(test_bit(FSCACHE_COOKIE_RELINQUISHED, &cookie->flags),
+		 "Trying to use relinquished cookie\n"))
+		return;
+
+	spin_lock(&cookie->lock);
+
+	atomic_inc(&cookie->n_active);
+
+again:
+	state = fscache_cookie_state(cookie);
+	switch (state) {
+	case FSCACHE_COOKIE_STATE_QUIESCENT:
+		queue = fscache_begin_lookup(cookie, will_modify);
+		break;
+
+	case FSCACHE_COOKIE_STATE_LOOKING_UP:
+	case FSCACHE_COOKIE_STATE_CREATING:
+		if (will_modify)
+			set_bit(FSCACHE_COOKIE_LOCAL_WRITE, &cookie->flags);
+		break;
+	case FSCACHE_COOKIE_STATE_ACTIVE:
+		if (will_modify &&
+		    !test_and_set_bit(FSCACHE_COOKIE_LOCAL_WRITE, &cookie->flags)) {
+			set_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
+			queue = true;
+		}
+		break;
+
+	case FSCACHE_COOKIE_STATE_FAILED:
+	case FSCACHE_COOKIE_STATE_WITHDRAWING:
+		break;
+
+	case FSCACHE_COOKIE_STATE_LRU_DISCARDING:
+		spin_unlock(&cookie->lock);
+		wait_var_event(&cookie->state,
+			       fscache_cookie_state(cookie) !=
+			       FSCACHE_COOKIE_STATE_LRU_DISCARDING);
+		spin_lock(&cookie->lock);
+		goto again;
+
+	case FSCACHE_COOKIE_STATE_DROPPED:
+	case FSCACHE_COOKIE_STATE_RELINQUISHING:
+		WARN(1, "Can't use cookie in state %u\n", state);
+		break;
+	}
+
+	spin_unlock(&cookie->lock);
+	if (queue)
+		fscache_queue_cookie(cookie, fscache_cookie_get_use_work);
+	_leave("");
+}
+EXPORT_SYMBOL(__fscache_use_cookie);
+
+static void fscache_unuse_cookie_locked(struct fscache_cookie *cookie)
+{
+	clear_bit(FSCACHE_COOKIE_DISABLED, &cookie->flags);
+	if (!test_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags))
+		return;
+
+	cookie->unused_at = jiffies;
+	spin_lock(&fscache_cookie_lru_lock);
+	if (list_empty(&cookie->commit_link)) {
+		fscache_get_cookie(cookie, fscache_cookie_get_lru);
+		fscache_stat(&fscache_n_cookies_lru);
+	}
+	list_move_tail(&cookie->commit_link, &fscache_cookie_lru);
+
+	spin_unlock(&fscache_cookie_lru_lock);
+	timer_reduce(&fscache_cookie_lru_timer,
+		     jiffies + fscache_lru_cookie_timeout);
+}
+
+/*
+ * Stop using the cookie for I/O.
+ */
+void __fscache_unuse_cookie(struct fscache_cookie *cookie,
+			    const void *aux_data, const loff_t *object_size)
+{
+	if (aux_data || object_size)
+		__fscache_update_cookie(cookie, aux_data, object_size);
+
+	if (atomic_dec_and_lock(&cookie->n_active, &cookie->lock)) {
+		fscache_unuse_cookie_locked(cookie);
+		spin_unlock(&cookie->lock);
+	}
+}
+EXPORT_SYMBOL(__fscache_unuse_cookie);
+
 /*
  * Perform work upon the cookie, such as committing its cache state,
  * relinquishing it or withdrawing the backing cache.  We're protected from the
@@ -542,6 +677,12 @@ static void fscache_cookie_state_machine(struct fscache_cookie *cookie)
 			fscache_prepare_to_write(cookie);
 			spin_lock(&cookie->lock);
 		}
+		if (test_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags)) {
+			__fscache_set_cookie_state(cookie,
+						   FSCACHE_COOKIE_STATE_LRU_DISCARDING);
+			wake = true;
+			goto again_locked;
+		}
 		fallthrough;
 
 	case FSCACHE_COOKIE_STATE_FAILED:
@@ -561,6 +702,7 @@ static void fscache_cookie_state_machine(struct fscache_cookie *cookie)
 		}
 		break;
 
+	case FSCACHE_COOKIE_STATE_LRU_DISCARDING:
 	case FSCACHE_COOKIE_STATE_RELINQUISHING:
 	case FSCACHE_COOKIE_STATE_WITHDRAWING:
 		if (cookie->cache_priv) {
@@ -577,6 +719,9 @@ static void fscache_cookie_state_machine(struct fscache_cookie *cookie)
 						   FSCACHE_COOKIE_STATE_DROPPED);
 			wake = true;
 			goto out;
+		case FSCACHE_COOKIE_STATE_LRU_DISCARDING:
+			fscache_see_cookie(cookie, fscache_cookie_see_lru_discard);
+			break;
 		case FSCACHE_COOKIE_STATE_WITHDRAWING:
 			fscache_see_cookie(cookie, fscache_cookie_see_withdraw);
 			break;
@@ -639,6 +784,76 @@ static void __fscache_withdraw_cookie(struct fscache_cookie *cookie)
 		fscache_queue_cookie(cookie, fscache_cookie_get_end_access);
 }
 
+static void fscache_cookie_lru_do_one(struct fscache_cookie *cookie)
+{
+	fscache_see_cookie(cookie, fscache_cookie_see_lru_do_one);
+
+	spin_lock(&cookie->lock);
+	if (cookie->state != FSCACHE_COOKIE_STATE_ACTIVE ||
+	    time_before(jiffies, cookie->unused_at + fscache_lru_cookie_timeout) ||
+	    atomic_read(&cookie->n_active) > 0) {
+		spin_unlock(&cookie->lock);
+		fscache_stat(&fscache_n_cookies_lru_removed);
+	} else {
+		set_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags);
+		spin_unlock(&cookie->lock);
+		fscache_stat(&fscache_n_cookies_lru_expired);
+		_debug("lru c=%x", cookie->debug_id);
+		__fscache_withdraw_cookie(cookie);
+	}
+
+	fscache_put_cookie(cookie, fscache_cookie_put_lru);
+}
+
+static void fscache_cookie_lru_worker(struct work_struct *work)
+{
+	struct fscache_cookie *cookie;
+	unsigned long unused_at;
+
+	spin_lock(&fscache_cookie_lru_lock);
+
+	while (!list_empty(&fscache_cookie_lru)) {
+		cookie = list_first_entry(&fscache_cookie_lru,
+					  struct fscache_cookie, commit_link);
+		unused_at = cookie->unused_at + fscache_lru_cookie_timeout;
+		if (time_before(jiffies, unused_at)) {
+			timer_reduce(&fscache_cookie_lru_timer, unused_at);
+			break;
+		}
+
+		list_del_init(&cookie->commit_link);
+		fscache_stat_d(&fscache_n_cookies_lru);
+		spin_unlock(&fscache_cookie_lru_lock);
+		fscache_cookie_lru_do_one(cookie);
+		spin_lock(&fscache_cookie_lru_lock);
+	}
+
+	spin_unlock(&fscache_cookie_lru_lock);
+}
+
+static void fscache_cookie_lru_timed_out(struct timer_list *timer)
+{
+	queue_work(fscache_wq, &fscache_cookie_lru_work);
+}
+
+static void fscache_cookie_drop_from_lru(struct fscache_cookie *cookie)
+{
+	bool need_put = false;
+
+	if (!list_empty(&cookie->commit_link)) {
+		spin_lock(&fscache_cookie_lru_lock);
+		if (!list_empty(&cookie->commit_link)) {
+			list_del_init(&cookie->commit_link);
+			fscache_stat_d(&fscache_n_cookies_lru);
+			fscache_stat(&fscache_n_cookies_lru_dropped);
+			need_put = true;
+		}
+		spin_unlock(&fscache_cookie_lru_lock);
+		if (need_put)
+			fscache_put_cookie(cookie, fscache_cookie_put_lru);
+	}
+}
+
 /*
  * Remove a cookie from the hash table.
  */
@@ -659,6 +874,7 @@ static void fscache_unhash_cookie(struct fscache_cookie *cookie)
 
 static void fscache_drop_withdraw_cookie(struct fscache_cookie *cookie)
 {
+	fscache_cookie_drop_from_lru(cookie);
 	__fscache_withdraw_cookie(cookie);
 }
 

fs/fscache/internal.h

@@ -57,6 +57,7 @@ static inline bool fscache_set_cache_state_maybe(struct fscache_cache *cache,
  */
 extern struct kmem_cache *fscache_cookie_jar;
 extern const struct seq_operations fscache_cookies_seq_ops;
+extern struct timer_list fscache_cookie_lru_timer;
 
 extern void fscache_print_cookie(struct fscache_cookie *cookie, char prefix);
 extern bool fscache_begin_cookie_access(struct fscache_cookie *cookie,
@@ -95,6 +96,10 @@ extern atomic_t fscache_n_volumes;
 extern atomic_t fscache_n_volumes_collision;
 extern atomic_t fscache_n_volumes_nomem;
 extern atomic_t fscache_n_cookies;
+extern atomic_t fscache_n_cookies_lru;
+extern atomic_t fscache_n_cookies_lru_expired;
+extern atomic_t fscache_n_cookies_lru_removed;
+extern atomic_t fscache_n_cookies_lru_dropped;
 
 extern atomic_t fscache_n_acquires;
 extern atomic_t fscache_n_acquires_ok;

fs/fscache/stats.c

@@ -17,6 +17,10 @@ atomic_t fscache_n_volumes;
 atomic_t fscache_n_volumes_collision;
 atomic_t fscache_n_volumes_nomem;
 atomic_t fscache_n_cookies;
+atomic_t fscache_n_cookies_lru;
+atomic_t fscache_n_cookies_lru_expired;
+atomic_t fscache_n_cookies_lru_removed;
+atomic_t fscache_n_cookies_lru_dropped;
 
 atomic_t fscache_n_acquires;
 atomic_t fscache_n_acquires_ok;
@@ -47,6 +51,14 @@ int fscache_stats_show(struct seq_file *m, void *v)
 		   atomic_read(&fscache_n_acquires_ok),
 		   atomic_read(&fscache_n_acquires_oom));
 
+	seq_printf(m, "LRU    : n=%u exp=%u rmv=%u drp=%u at=%ld\n",
+		   atomic_read(&fscache_n_cookies_lru),
+		   atomic_read(&fscache_n_cookies_lru_expired),
+		   atomic_read(&fscache_n_cookies_lru_removed),
+		   atomic_read(&fscache_n_cookies_lru_dropped),
+		   timer_pending(&fscache_cookie_lru_timer) ?
+		   fscache_cookie_lru_timer.expires - jiffies : 0);
+
 	seq_printf(m, "Updates: n=%u\n",
 		   atomic_read(&fscache_n_updates));