2020-09-16 19:47:37

by Ye Bin

Subject: [PATCH v5 2/2] 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.
Ritesh Harjani suggest to in ext4_mb_discard_group_preallocations function we
only increase discard_pa_seq when there is some PA to free.

Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
Signed-off-by: Ye Bin <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Ritesh Harjani <[email protected]>
---
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 f736819a381b..4d40d8dc518c 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4189,7 +4189,6 @@ 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);
@@ -4206,6 +4205,9 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
/* seems this one can be freed ... */
ext4_mb_mark_pa_deleted(sb, pa);

+ if (!free)
+ this_cpu_inc(discard_pa_seq);
+
/* we can trust pa_free ... */
free += pa->pa_free;

--
2.25.4

2020-09-24 15:03:07

by Theodore Ts'o

Subject: Re: [PATCH v5 2/2] ext4: Fix dead loop in ext4_mb_new_blocks

On Wed, Sep 16, 2020 at 07:38:59PM +0800, Ye Bin wrote:
> 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.
> Ritesh Harjani suggest to in ext4_mb_discard_group_preallocations function we
> only increase discard_pa_seq when there is some PA to free.
>
> Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
> Signed-off-by: Ye Bin <[email protected]>
> Reviewed-by: Jan Kara <[email protected]>
> Reviewed-by: Ritesh Harjani <[email protected]>

Thanks, applied.

- Ted