ext4: discard preallocations before releasing group lock

ext4_mb_discard_group_preallocations() can be releasing group lock with
preallocations accumulated on its local list. Thus although
discard_pa_seq was incremented and concurrent allocating processes will
be retrying allocations, it can happen that premature ENOSPC error is
returned because blocks used for preallocations are not available for
reuse yet. Make sure we always free locally accumulated preallocations
before releasing group lock.

Fixes: 07b5b8e1 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200924150959.4335-1-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -4160,7 +4160,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 	struct ext4_buddy e4b;
 	int err;
 	int busy = 0;
-	int free = 0;
+	int free, free_total = 0;
 
 	mb_debug(sb, "discard preallocation for group %u\n", group);
 	if (list_empty(&grp->bb_prealloc_list))
@@ -4188,6 +4188,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 
 	INIT_LIST_HEAD(&list);
 repeat:
+	free = 0;
 	ext4_lock_group(sb, group);
 	list_for_each_entry_safe(pa, tmp,
 				&grp->bb_prealloc_list, pa_group_list) {
@@ -4217,22 +4218,6 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 		list_add(&pa->u.pa_tmp_list, &list);
 	}
 
-	/* if we still need more blocks and some PAs were used, try again */
-	if (free < needed && busy) {
-		busy = 0;
-		ext4_unlock_group(sb, group);
-		cond_resched();
-		goto repeat;
-	}
-
-	/* found anything to free? */
-	if (list_empty(&list)) {
-		BUG_ON(free != 0);
-		mb_debug(sb, "Someone else may have freed PA for this group %u\n",
-			 group);
-		goto out;
-	}
-
 	/* now free all selected PAs */
 	list_for_each_entry_safe(pa, tmp, &list, u.pa_tmp_list) {
 
@@ -4250,14 +4235,22 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 		call_rcu(&(pa)->u.pa_rcu, ext4_mb_pa_callback);
 	}
 
-out:
+	free_total += free;
+
+	/* if we still need more blocks and some PAs were used, try again */
+	if (free_total < needed && busy) {
+		ext4_unlock_group(sb, group);
+		cond_resched();
+		busy = 0;
+		goto repeat;
+	}
 	ext4_unlock_group(sb, group);
 	ext4_mb_unload_buddy(&e4b);
 	put_bh(bitmap_bh);
 out_dbg:
 	mb_debug(sb, "discarded (%d) blocks preallocated for group %u bb_free (%d)\n",
-		 free, group, grp->bb_free);
-	return free;
+		 free_total, group, grp->bb_free);
+	return free_total;
 }
 
 /*