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Message-ID: <7e268dff-4f29-4155-8644-45be74d4c465@mgml.me>
Date: Mon, 1 Sep 2025 13:22:40 +0900
From: Kenta Akagi <k@...l.me>
To: Li Nan <linan666@...weicloud.com>, Song Liu <song@...nel.org>,
Yu Kuai <yukuai3@...wei.com>, Mariusz Tkaczyk <mtkaczyk@...nel.org>,
Guoqing Jiang <jgq516@...il.com>
Cc: linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
Kenta Akagi <k@...l.me>
Subject: Re: [PATCH v3 1/3] md/raid1,raid10: Do not set MD_BROKEN on failfast
io failure
On 2025/09/01 12:22, Li Nan wrote:
>
>
> 在 2025/8/31 2:10, Kenta Akagi 写道:
>>
>>
>> On 2025/08/30 17:48, Li Nan wrote:
>>>
>>>
>>> 在 2025/8/29 20:21, Kenta Akagi 写道:
>>>>
>>>>
>>>> On 2025/08/29 11:54, Li Nan wrote:
>>>>>
>>>>> 在 2025/8/29 0:32, Kenta Akagi 写道:
>>>>>> This commit ensures that an MD_FAILFAST IO failure does not put
>>>>>> the array into a broken state.
>>>>>>
>>>>>> When failfast is enabled on rdev in RAID1 or RAID10,
>>>>>> the array may be flagged MD_BROKEN in the following cases.
>>>>>> - If MD_FAILFAST IOs to multiple rdevs fail simultaneously
>>>>>> - If an MD_FAILFAST metadata write to the 'last' rdev fails
>>>>>
>>>>> [...]
>>>>>
>>>>>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>>>>>> index 408c26398321..8a61fd93b3ff 100644
>>>>>> --- a/drivers/md/raid1.c
>>>>>> +++ b/drivers/md/raid1.c
>>>>>> @@ -470,6 +470,7 @@ static void raid1_end_write_request(struct bio *bio)
>>>>>> (bio->bi_opf & MD_FAILFAST) &&
>>>>>> /* We never try FailFast to WriteMostly devices */
>>>>>> !test_bit(WriteMostly, &rdev->flags)) {
>>>>>> + set_bit(FailfastIOFailure, &rdev->flags);
>>>>>> md_error(r1_bio->mddev, rdev);
>>>>>> }
>>>>>> @@ -1746,8 +1747,12 @@ static void raid1_status(struct seq_file *seq, struct mddev *mddev)
>>>>>> * - recovery is interrupted.
>>>>>> * - &mddev->degraded is bumped.
>>>>>> *
>>>>>> - * @rdev is marked as &Faulty excluding case when array is failed and
>>>>>> - * &mddev->fail_last_dev is off.
>>>>>> + * If @rdev has &FailfastIOFailure and it is the 'last' rdev,
>>>>>> + * then @mddev and @rdev will not be marked as failed.
>>>>>> + *
>>>>>> + * @rdev is marked as &Faulty excluding any cases:
>>>>>> + * - when @mddev is failed and &mddev->fail_last_dev is off
>>>>>> + * - when @rdev is last device and &FailfastIOFailure flag is set
>>>>>> */
>>>>>> static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
>>>>>> {
>>>>>> @@ -1758,6 +1763,13 @@ static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
>>>>>> if (test_bit(In_sync, &rdev->flags) &&
>>>>>> (conf->raid_disks - mddev->degraded) == 1) {
>>>>>> + if (test_and_clear_bit(FailfastIOFailure, &rdev->flags)) {
>>>>>> + spin_unlock_irqrestore(&conf->device_lock, flags);
>>>>>> + pr_warn_ratelimited("md/raid1:%s: Failfast IO failure on %pg, "
>>>>>> + "last device but ignoring it\n",
>>>>>> + mdname(mddev), rdev->bdev);
>>>>>> + return;
>>>>>> + }
>>>>>> set_bit(MD_BROKEN, &mddev->flags);
>>>>>> if (!mddev->fail_last_dev) {
>>>>>> @@ -2148,6 +2160,7 @@ static int fix_sync_read_error(struct r1bio *r1_bio)
>>>>>> if (test_bit(FailFast, &rdev->flags)) {
>>>>>> /* Don't try recovering from here - just fail it
>>>>>> * ... unless it is the last working device of course */
>>>>>> + set_bit(FailfastIOFailure, &rdev->flags);
>>>>>> md_error(mddev, rdev);
>>>>>> if (test_bit(Faulty, &rdev->flags))
>>>>>> /* Don't try to read from here, but make sure
>>>>>> @@ -2652,6 +2665,7 @@ static void handle_read_error(struct r1conf *conf, struct r1bio *r1_bio)
>>>>>> fix_read_error(conf, r1_bio);
>>>>>> unfreeze_array(conf);
>>>>>> } else if (mddev->ro == 0 && test_bit(FailFast, &rdev->flags)) {
>>>>>> + set_bit(FailfastIOFailure, &rdev->flags);
>>>>>> md_error(mddev, rdev);
>>>>>> } else {
>>>>>> r1_bio->bios[r1_bio->read_disk] = IO_BLOCKED;
>>>>>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>>>>>> index b60c30bfb6c7..530ad6503189 100644
>>>>>> --- a/drivers/md/raid10.c
>>>>>> +++ b/drivers/md/raid10.c
>>>>>> @@ -488,6 +488,7 @@ static void raid10_end_write_request(struct bio *bio)
>>>>>> dec_rdev = 0;
>>>>>> if (test_bit(FailFast, &rdev->flags) &&
>>>>>> (bio->bi_opf & MD_FAILFAST)) {
>>>>>> + set_bit(FailfastIOFailure, &rdev->flags);
>>>>>> md_error(rdev->mddev, rdev);
>>>>>> }
>>>>>>
>>>>>
>>>>> Thank you for the patch. There may be an issue with 'test_and_clear'.
>>>>>
>>>>> If two write IO go to the same rdev, MD_BROKEN may be set as below:
>>>>
>>>>> IO1 IO2
>>>>> set FailfastIOFailure
>>>>> set FailfastIOFailure
>>>>> md_error
>>>>> raid1_error
>>>>> test_and_clear FailfastIOFailur
>>>>> md_error
>>>>> raid1_error
>>>>> //FailfastIOFailur is cleared
>>>>> set MD_BROKEN
>>>>>
>>>>> Maybe we should check whether FailfastIOFailure is already set before
>>>>> setting it. It also needs to be considered in metadata writes.
>>>> Thank you for reviewing.
>>>>
>>>> I agree, this seems to be as you described.
>>>> So, should it be implemented as follows?
>>>>
>>>> bool old=false;
>>>> do{
>>>> spin_lock_irqsave(&conf->device_lock, flags);
>>>> old = test_and_set_bit(FailfastIOFailure, &rdev->flags);
>>>> spin_unlock_irqrestore(&conf->device_lock, flags);
>>>> }while(old);
>>>>
>>>> However, since I am concerned about potential deadlocks,
>>>> so I am considering two alternative approaches:
>>>>
>>>> * Add an atomic_t counter to md_rdev to track failfast IO failures.
>>>>
>>>> This may set MD_BROKEN at a slightly incorrect timing, but mixing
>>>> error handling of Failfast and non-Failfast IOs appears to be rare.
>>>> In any case, the final outcome would be the same, i.e. the array
>>>> ends up with MD_BROKEN. Therefore, I think this should not cause
>>>> issues. I think the same applies to test_and_set_bit.
>>>>
>>>> IO1 IO2 IO3
>>>> FailfastIOFailure Normal IOFailure FailfastIOFailure
>>>> atomic_inc
>>>> md_error atomic_inc
>>>> raid1_error
>>>> atomic_dec //2to1
>>>> md_error
>>>> raid1_error md_error
>>>> atomic_dec //1to0 raid1_error
>>>> atomic_dec //0
>>>> set MD_BROKEN
>>>>
>>>> * Alternatively, create a separate error handler,
>>>> e.g. md_error_failfast(), that clearly does not fail the array.
>>>>
>>>> This approach would require somewhat larger changes and may not
>>>> be very elegant, but it seems to be a reliable way to ensure
>>>> MD_BROKEN is never set at the wrong timing.
>>>>
>>>> Which of these three approaches would you consider preferable?
>>>> I would appreciate your feedback.
>>>>
>>>>
>>>> For metadata writes, I plan to clear_bit MD_FAILFAST_SUPPORTED
>>>> when the array is degraded.
>>>>
>>>> Thanks,
>>>> Akagi
>>>>
>>>
>>> I took a closer look at the FailFast code and found a few issues, using
>>> RAID1 as an example:
>>>
>>> For normal read/write IO, FailFast is only triggered when there is another
>>> disk is available, as seen in read_balance() and raid1_write_request().
>>> In raid1_error(), MD_BROKEN is set only when no other disks are available.
>>
>> Hi,
>> Agree, I think so too.
>>
>>> So, the FailFast for normal read/write is not triggered in the scenario you
>>> described in cover-letter.
>>
>> This corresponds to the case described in the commit message of PATCH v3 1/3.
>> "Normally, MD_FAILFAST IOs are not issued to the 'last' rdev, so this is
>> an edge case; however, it can occur if rdevs in a non-degraded
>> array share the same path and that path is lost, or if a metadata
>> write is triggered in a degraded array and fails due to failfast."
>>
>> To describe it in more detail, the flow is as follows:
>>
>> Prerequisites:
>>
>> - Both rdevs are in-sync
>> - Both rdevs have in-flight MD_FAILFAST bios
>> - Both rdevs depend on the same lower-level path
>> (e.g., nvme-tcp over a single Ethernet interface)
>>
>> Sequence:
>>
>> - A bios with REQ_FAILFAST_DEV fails (e.g., due to a temporary network outage),
>> in the case of nvme-tcp:
>> - The Ethernet connection is lost on the node where md is running over 5 seconds
>> - Then the connection is restored. Idk the details of nvme-tcp implementation,
>> but it seems that failfast IOs finish only after the connection is back.
>> - All failfast bios fail, raid1_end_write_request is called.
>> - md_error() marks one rdev Faulty; the other rdev becomes the 'last' rdev.
>> - md_error() on the last rdev sets MD_BROKEN on the array - fail_last_dev=1 is unlikely.
>> - The write is retried via handle_write_finished -> narrow_write_error, usually succeeding.
>> - MD_BROKEN remains set, leaving the array in a state where no further writes can occur.
>>
>
> Thanks for your patient explanation. I understand. Maybe we need a separate
> error-handling path for failfast. How about adding an extra parameter to md_error()?
Thank you for reviewing.
I am thinking of proceeding like as follows.
md_error is EXPORT_SYMBOL. I think that it is undesirable to change the ABI of this function.
...
diff --git a/drivers/md/md.c b/drivers/md/md.c
index ac85ec73a409..855cddeb0c09 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8197,3 +8197,3 @@ EXPORT_SYMBOL(md_unregister_thread);
-void md_error(struct mddev *mddev, struct md_rdev *rdev)
+void _md_error(struct mddev *mddev, struct md_rdev *rdev, bool nofail)
{
@@ -8204,3 +8204,3 @@ void md_error(struct mddev *mddev, struct md_rdev *rdev)
return;
- mddev->pers->error_handler(mddev, rdev);
+ mddev->pers->error_handler(mddev, rdev, nofail);
@@ -8222,4 +8222,26 @@ void md_error(struct mddev *mddev, struct md_rdev *rdev)
}
+
+void md_error(struct mddev *mddev, struct md_rdev *rdev)
+{
+ return _md_error(mddev, rdev, false);
+}
EXPORT_SYMBOL(md_error);
+void md_error_failfast(struct mddev *mddev, struct md_rdev *rdev)
+{
+ WARN_ON(mddev->pers->head.id != ID_RAID1 &&
+ mddev->pers->head.id != ID_RAID10);
+ return _md_error(mddev, rdev, true);
+}
+EXPORT_SYMBOL(md_error_failfast);
+
/* seq_file implementation /proc/mdstat */
diff --git a/drivers/md/md.h b/drivers/md/md.h
index 51af29a03079..6ca1aea630ce 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -758,3 +758,3 @@ struct md_personality
*/
- void (*error_handler)(struct mddev *mddev, struct md_rdev *rdev);
+ void (*error_handler)(struct mddev *mddev, struct md_rdev *rdev, bool nofail);
int (*hot_add_disk) (struct mddev *mddev, struct md_rdev *rdev);
@@ -903,3 +903,5 @@ extern void md_write_end(struct mddev *mddev);
extern void md_done_sync(struct mddev *mddev, int blocks, int ok);
+void _md_error(struct mddev *mddev, struct md_rdev *rdev, bool nofail);
extern void md_error(struct mddev *mddev, struct md_rdev *rdev);
+extern void md_error_failfast(struct mddev *mddev, struct md_rdev *rdev);
extern void md_finish_reshape(struct mddev *mddev);
diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index f1d8811a542a..8aea51227a96 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c
@@ -637,3 +637,4 @@ static void raid0_status(struct seq_file *seq, struct mddev *mddev)
-static void raid0_error(struct mddev *mddev, struct md_rdev *rdev)
+static void raid0_error(struct mddev *mddev, struct md_rdev *rdev,
+ bool nofail __maybe_unused)
{
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 408c26398321..d93275899e9e 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1739,2 +1739,3 @@ static void raid1_status(struct seq_file *seq, struct mddev *mddev)
* @rdev: member device to fail.
+ * @nofail: @mdev and @rdev must not fail even if @rdev is the last when @nofail set
*
@@ -1748,6 +1749,8 @@ static void raid1_status(struct seq_file *seq, struct mddev *mddev)
*
- * @rdev is marked as &Faulty excluding case when array is failed and
- * &mddev->fail_last_dev is off.
+ * @rdev is marked as &Faulty excluding any cases:
+ * - when @mddev is failed and &mddev->fail_last_dev is off
+ * - when @rdev is last device and @nofail is true
*/
-static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
+static void raid1_error(struct mddev *mddev, struct md_rdev *rdev,
+ bool nofail)
{
@@ -1760,2 +1763,9 @@ static void raid1_error(struct mddev *mddev, struct md_rdev *rdev)
(conf->raid_disks - mddev->degraded) == 1) {
+ if (nofail) {
+ spin_unlock_irqrestore(&conf->device_lock, flags);
+ pr_warn_ratelimited("md/raid1:%s: IO failure on %pg, "
+ "last device but ignoring it\n",
+ mdname(mddev), rdev->bdev);
+ return;
+ }
set_bit(MD_BROKEN, &mddev->flags);
...
> Kuai, do you have any suggestions?
>
>>> Normal writes may call md_error() in narrow_write_error. Normal reads do
>>> not execute md_error() on the last disk.
>>>
>>> Perhaps you should get more information to confirm how MD_BROKEN is set in
>>> normal read/write IO.
>>
>> Should I add the above sequence of events to the cover letter, or commit message?
>>
>
> I think we should mention this in the commit message.
Understood. I will explicitly describe this in the commit message in v4.
Thanks,
Akagi
>> Thanks,
>> Akagi
>>
>>> --
>>> Thanks,
>>> Nan
>>>
>>>
>>
>>
>> .
>
> --
> Thanks,
> Nan
>
>
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