linux-ext4 - [PATCH v3] ext4: Fix dead loop in ext4_mb_new

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Date:   Thu, 10 Sep 2020 17:12:52 +0800
From:   Ye Bin <yebin10@...wei.com>
To:     <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.com>,
        <linux-ext4@...r.kernel.org>
CC:     Ye Bin <yebin10@...wei.com>
Subject: [PATCH v3] ext4: Fix dead loop in ext4_mb_new_blocks

As we test disk offline/online with running fsstress, we find fsstress
process is keeping running state.
kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114
....
kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114

ext4_mb_new_blocks
repeat:
	ext4_mb_discard_preallocations_should_retry(sb, ac, &seq)
		freed = ext4_mb_discard_preallocations
			ext4_mb_discard_group_preallocations
				this_cpu_inc(discard_pa_seq);
		---> freed == 0
		seq_retry = ext4_get_discard_pa_seq_sum
			for_each_possible_cpu(__cpu)
				__seq += per_cpu(discard_pa_seq, __cpu);
		if (seq_retry != *seq) {
			*seq = seq_retry;
			ret = true;
		}

As we see seq_retry is sum of discard_pa_seq every cpu, if
ext4_mb_discard_group_preallocations return zero discard_pa_seq in this
cpu maybe increase one, so condition "seq_retry != *seq" have always
been met.
To Fix this problem, in ext4_mb_discard_group_preallocations function increase
discard_pa_seq only when it found preallocation to discard.

Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
Signed-off-by: Ye Bin <yebin10@...wei.com>
---
 fs/ext4/mballoc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index f386fe62727d..fd55264dc3fe 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4191,7 +4191,6 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 	INIT_LIST_HEAD(&list);
 repeat:
 	ext4_lock_group(sb, group);
-	this_cpu_inc(discard_pa_seq);
 	list_for_each_entry_safe(pa, tmp,
 				&grp->bb_prealloc_list, pa_group_list) {
 		spin_lock(&pa->pa_lock);
@@ -4233,6 +4232,9 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 		goto out;
 	}
 
+	/* only increase when find reallocation to discard */
+	this_cpu_inc(discard_pa_seq);
+
 	/* now free all selected PAs */
 	list_for_each_entry_safe(pa, tmp, &list, u.pa_tmp_list) {
 
-- 
2.25.4