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Merge tag 'fs_for_v6.6-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs

Pull quota regression fix from Jan Kara.

* tag 'fs_for_v6.6-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs:
  quota: Fix slow quotaoff

fs/quota/dquot.c

@@ -233,19 +233,18 @@ static void put_quota_format(struct quota_format_type *fmt)
  * All dquots are placed to the end of inuse_list when first created, and this
  * list is used for invalidate operation, which must look at every dquot.
  *
- * When the last reference of a dquot will be dropped, the dquot will be
- * added to releasing_dquots. We'd then queue work item which would call
+ * When the last reference of a dquot is dropped, the dquot is added to
+ * releasing_dquots. We'll then queue work item which will call
  * synchronize_srcu() and after that perform the final cleanup of all the
- * dquots on the list. Both releasing_dquots and free_dquots use the
- * dq_free list_head in the dquot struct. When a dquot is removed from
- * releasing_dquots, a reference count is always subtracted, and if
- * dq_count == 0 at that point, the dquot will be added to the free_dquots.
+ * dquots on the list. Each cleaned up dquot is moved to free_dquots list.
+ * Both releasing_dquots and free_dquots use the dq_free list_head in the dquot
+ * struct.
  *
- * Unused dquots (dq_count == 0) are added to the free_dquots list when freed,
- * and this list is searched whenever we need an available dquot.  Dquots are
- * removed from the list as soon as they are used again, and
- * dqstats.free_dquots gives the number of dquots on the list. When
- * dquot is invalidated it's completely released from memory.
+ * Unused and cleaned up dquots are in the free_dquots list and this list is
+ * searched whenever we need an available dquot. Dquots are removed from the
+ * list as soon as they are used again and dqstats.free_dquots gives the number
+ * of dquots on the list. When dquot is invalidated it's completely released
+ * from memory.
  *
  * Dirty dquots are added to the dqi_dirty_list of quota_info when mark
  * dirtied, and this list is searched when writing dirty dquots back to
@@ -321,6 +320,7 @@ static inline void put_dquot_last(struct dquot *dquot)
 static inline void put_releasing_dquots(struct dquot *dquot)
 {
 	list_add_tail(&dquot->dq_free, &releasing_dquots);
+	set_bit(DQ_RELEASING_B, &dquot->dq_flags);
 }
 
 static inline void remove_free_dquot(struct dquot *dquot)
@@ -328,8 +328,10 @@ static inline void remove_free_dquot(struct dquot *dquot)
 	if (list_empty(&dquot->dq_free))
 		return;
 	list_del_init(&dquot->dq_free);
-	if (!atomic_read(&dquot->dq_count))
+	if (!test_bit(DQ_RELEASING_B, &dquot->dq_flags))
 		dqstats_dec(DQST_FREE_DQUOTS);
+	else
+		clear_bit(DQ_RELEASING_B, &dquot->dq_flags);
 }
 
 static inline void put_inuse(struct dquot *dquot)
@@ -581,12 +583,6 @@ restart:
 			continue;
 		/* Wait for dquot users */
 		if (atomic_read(&dquot->dq_count)) {
-			/* dquot in releasing_dquots, flush and retry */
-			if (!list_empty(&dquot->dq_free)) {
-				spin_unlock(&dq_list_lock);
-				goto restart;
-			}
-
 			atomic_inc(&dquot->dq_count);
 			spin_unlock(&dq_list_lock);
 			/*
@@ -605,6 +601,15 @@ restart:
 			 * restart. */
 			goto restart;
 		}
+		/*
+		 * The last user already dropped its reference but dquot didn't
+		 * get fully cleaned up yet. Restart the scan which flushes the
+		 * work cleaning up released dquots.
+		 */
+		if (test_bit(DQ_RELEASING_B, &dquot->dq_flags)) {
+			spin_unlock(&dq_list_lock);
+			goto restart;
+		}
 		/*
 		 * Quota now has no users and it has been written on last
 		 * dqput()
@@ -696,6 +701,13 @@ int dquot_writeback_dquots(struct super_block *sb, int type)
 						 dq_dirty);
 
 			WARN_ON(!dquot_active(dquot));
+			/* If the dquot is releasing we should not touch it */
+			if (test_bit(DQ_RELEASING_B, &dquot->dq_flags)) {
+				spin_unlock(&dq_list_lock);
+				flush_delayed_work(&quota_release_work);
+				spin_lock(&dq_list_lock);
+				continue;
+			}
 
 			/* Now we have active dquot from which someone is
  			 * holding reference so we can safely just increase
@@ -809,18 +821,18 @@ static void quota_release_workfn(struct work_struct *work)
 	/* Exchange the list head to avoid livelock. */
 	list_replace_init(&releasing_dquots, &rls_head);
 	spin_unlock(&dq_list_lock);
+	synchronize_srcu(&dquot_srcu);
 
 restart:
-	synchronize_srcu(&dquot_srcu);
 	spin_lock(&dq_list_lock);
 	while (!list_empty(&rls_head)) {
 		dquot = list_first_entry(&rls_head, struct dquot, dq_free);
-		/* Dquot got used again? */
-		if (atomic_read(&dquot->dq_count) > 1) {
-			remove_free_dquot(dquot);
-			atomic_dec(&dquot->dq_count);
-			continue;
-		}
+		WARN_ON_ONCE(atomic_read(&dquot->dq_count));
+		/*
+		 * Note that DQ_RELEASING_B protects us from racing with
+		 * invalidate_dquots() calls so we are safe to work with the
+		 * dquot even after we drop dq_list_lock.
+		 */
 		if (dquot_dirty(dquot)) {
 			spin_unlock(&dq_list_lock);
 			/* Commit dquot before releasing */
@@ -834,7 +846,6 @@ restart:
 		}
 		/* Dquot is inactive and clean, now move it to free list */
 		remove_free_dquot(dquot);
-		atomic_dec(&dquot->dq_count);
 		put_dquot_last(dquot);
 	}
 	spin_unlock(&dq_list_lock);
@@ -875,6 +886,7 @@ void dqput(struct dquot *dquot)
 	BUG_ON(!list_empty(&dquot->dq_free));
 #endif
 	put_releasing_dquots(dquot);
+	atomic_dec(&dquot->dq_count);
 	spin_unlock(&dq_list_lock);
 	queue_delayed_work(system_unbound_wq, &quota_release_work, 1);
 }
@@ -963,7 +975,7 @@ we_slept:
 		dqstats_inc(DQST_LOOKUPS);
 	}
 	/* Wait for dq_lock - after this we know that either dquot_release() is
-	 * already finished or it will be canceled due to dq_count > 1 test */
+	 * already finished or it will be canceled due to dq_count > 0 test */
 	wait_on_dquot(dquot);
 	/* Read the dquot / allocate space in quota file */
 	if (!dquot_active(dquot)) {

include/linux/quota.h

@@ -285,7 +285,9 @@ static inline void dqstats_dec(unsigned int type)
 #define DQ_FAKE_B	3	/* no limits only usage */
 #define DQ_READ_B	4	/* dquot was read into memory */
 #define DQ_ACTIVE_B	5	/* dquot is active (dquot_release not called) */
-#define DQ_LASTSET_B	6	/* Following 6 bits (see QIF_) are reserved\
+#define DQ_RELEASING_B	6	/* dquot is in releasing_dquots list waiting
+				 * to be cleaned up */
+#define DQ_LASTSET_B	7	/* Following 6 bits (see QIF_) are reserved\
 				 * for the mask of entries set via SETQUOTA\
 				 * quotactl. They are set under dq_data_lock\
 				 * and the quota format handling dquot can\

include/linux/quotaops.h

@@ -57,7 +57,7 @@ static inline bool dquot_is_busy(struct dquot *dquot)
 {
 	if (test_bit(DQ_MOD_B, &dquot->dq_flags))
 		return true;
-	if (atomic_read(&dquot->dq_count) > 1)
+	if (atomic_read(&dquot->dq_count) > 0)
 		return true;
 	return false;
 }