ext4: flush background discard kwork when retry allocation

The background discard kwork tries to mark blocks used and issue
discard. This can make filesystem suffer from NOSPC error, xfstest
generic/371 can fail due to it. Fix it by flushing discard kwork
in ext4_should_retry_alloc. At the same time, give up discard at
the moment.
Signed-off-by: Wang Jianchao <wangjianchao@kuaishou.com>
Link: https://lore.kernel.org/r/20210830075246.12516-6-jianchao.wan9@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/balloc.c

@@ -652,8 +652,14 @@ int ext4_should_retry_alloc(struct super_block *sb, int *retries)
 	 * possible we just missed a transaction commit that did so
 	 */
 	smp_mb();
-	if (sbi->s_mb_free_pending == 0)
+	if (sbi->s_mb_free_pending == 0) {
+		if (test_opt(sb, DISCARD)) {
+			atomic_inc(&sbi->s_retry_alloc_pending);
+			flush_work(&sbi->s_discard_work);
+			atomic_dec(&sbi->s_retry_alloc_pending);
+		}
 		return ext4_has_free_clusters(sbi, 1, 0);
+	}
 
 	/*
 	 * it's possible we've just missed a transaction commit here,

fs/ext4/ext4.h

@@ -1538,6 +1538,7 @@ struct ext4_sb_info {
 						   after commit completed */
 	struct list_head s_discard_list;
 	struct work_struct s_discard_work;
+	atomic_t s_retry_alloc_pending;
 	struct rb_root s_mb_avg_fragment_size_root;
 	rwlock_t s_mb_rb_lock;
 	struct list_head *s_mb_largest_free_orders;

fs/ext4/mballoc.c

@@ -3331,9 +3331,11 @@ static void ext4_discard_work(struct work_struct *work)
 	load_grp = UINT_MAX;
 	list_for_each_entry_safe(fd, nfd, &discard_list, efd_list) {
 		/*
-		 * If filesystem is umounting or no memory, give up the discard
+		 * If filesystem is umounting or no memory or suffering
+		 * from no space, give up the discard
 		 */
-		if ((sb->s_flags & SB_ACTIVE) && !err) {
+		if ((sb->s_flags & SB_ACTIVE) && !err &&
+		    !atomic_read(&sbi->s_retry_alloc_pending)) {
 			grp = fd->efd_group;
 			if (grp != load_grp) {
 				if (load_grp != UINT_MAX)
@@ -3431,6 +3433,7 @@ int ext4_mb_init(struct super_block *sb)
 	INIT_LIST_HEAD(&sbi->s_freed_data_list);
 	INIT_LIST_HEAD(&sbi->s_discard_list);
 	INIT_WORK(&sbi->s_discard_work, ext4_discard_work);
+	atomic_set(&sbi->s_retry_alloc_pending, 0);
 
 	sbi->s_mb_max_to_scan = MB_DEFAULT_MAX_TO_SCAN;
 	sbi->s_mb_min_to_scan = MB_DEFAULT_MIN_TO_SCAN;