Re: [PATCH 08/13] lightnvm: pblk: Set proper read stutus in bio

["Matias Bjørling" <mb@lightnvm.io>]

On 3/4/19 2:45 PM, Javier González wrote:
> 
>> On 4 Mar 2019, at 14.08, Matias Bjørling <mb@lightnvm.io> wrote:
>>
>> On 3/4/19 1:51 PM, Igor Konopko wrote:
>>> On 04.03.2019 13:14, Hans Holmberg wrote:
>>>> On Mon, Mar 4, 2019 at 10:48 AM Javier González <javier@javigon.com> wrote:
>>>>>> On 4 Mar 2019, at 10.35, Hans Holmberg <hans.ml.holmberg@owltronix.com> wrote:
>>>>>>
>>>>>> On Mon, Mar 4, 2019 at 9:03 AM Javier González <javier@javigon.com> wrote:
>>>>>>>> On 27 Feb 2019, at 18.14, Igor Konopko <igor.j.konopko@intel.com> wrote:
>>>>>>>>
>>>>>>>> Currently in case of read errors, bi_status is not
>>>>>>>> set properly which leads to returning inproper data
>>>>>>>> to higher layer. This patch fix that by setting proper
>>>>>>>> status in case of read errors
>>>>>>>>
>>>>>>>> Patch also removes unnecessary warn_once(), which does
>>>>>>>> not make sense in that place, since user bio is not used
>>>>>>>> for interation with drive and thus bi_status will not be
>>>>>>>> set here.
>>>>>>>>
>>>>>>>> Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
>>>>>>>> ---
>>>>>>>> drivers/lightnvm/pblk-read.c | 11 +++++------
>>>>>>>> 1 file changed, 5 insertions(+), 6 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/lightnvm/pblk-read.c b/drivers/lightnvm/pblk-read.c
>>>>>>>> index 3789185144da..39c1d6ccaedb 100644
>>>>>>>> --- a/drivers/lightnvm/pblk-read.c
>>>>>>>> +++ b/drivers/lightnvm/pblk-read.c
>>>>>>>> @@ -175,11 +175,10 @@ static void pblk_read_check_rand(struct pblk *pblk, struct nvm_rq *rqd,
>>>>>>>>        WARN_ONCE(j != rqd->nr_ppas, "pblk: corrupted random request\n");
>>>>>>>> }
>>>>>>>>
>>>>>>>> -static void pblk_end_user_read(struct bio *bio)
>>>>>>>> +static void pblk_end_user_read(struct bio *bio, int error)
>>>>>>>> {
>>>>>>>> -#ifdef CONFIG_NVM_PBLK_DEBUG
>>>>>>>> -     WARN_ONCE(bio->bi_status, "pblk: corrupted read bio\n");
>>>>>>>> -#endif
>>>>>>>> +     if (error && error != NVM_RSP_WARN_HIGHECC)
>>>>>>>> +             bio_io_error(bio);
>>>>>>>>        bio_endio(bio);
>>>>>>>> }
>>>>>>>>
>>>>>>>> @@ -219,7 +218,7 @@ static void pblk_end_io_read(struct nvm_rq *rqd)
>>>>>>>>        struct pblk_g_ctx *r_ctx = nvm_rq_to_pdu(rqd);
>>>>>>>>        struct bio *bio = (struct bio *)r_ctx->private;
>>>>>>>>
>>>>>>>> -     pblk_end_user_read(bio);
>>>>>>>> +     pblk_end_user_read(bio, rqd->error);
>>>>>>>>        __pblk_end_io_read(pblk, rqd, true);
>>>>>>>> }
>>>>>>>>
>>>>>>>> @@ -292,7 +291,7 @@ static void pblk_end_partial_read(struct nvm_rq *rqd)
>>>>>>>>        rqd->bio = NULL;
>>>>>>>>        rqd->nr_ppas = nr_secs;
>>>>>>>>
>>>>>>>> -     bio_endio(bio);
>>>>>>>> +     pblk_end_user_read(bio, rqd->error);
>>>>>>>>        __pblk_end_io_read(pblk, rqd, false);
>>>>>>>> }
>>>>>>>>
>>>>>>>> --
>>>>>>>> 2.17.1
>>>>>>>
>>>>>>> This is by design. We do not report the read errors as in any other
>>>>>>> block device - this is why we clone the read bio.
>>>>>>
>>>>>> Could you elaborate on why not reporting read errors is a good thing in pblk?
>>>>>
>>>>> Normal block devices do not report read errors on the completion path
>>>>> unless it is a fatal error. This is actually not well understood by the
>>>>> upper layers, which tend to assume that the device is completely broken.
>>>>
>>>> So returning bogus data without even a warning is a preferred
>>>> solution? You want to force "the upper layers" to do checksumming?
>>>>
>>>> It's fine to mask out NVM_RSP_WARN_HIGHECC, since that is just a
>>>> warning that OCSSD 2.0 adds. The data should still be good.
>>>> All other errors (see 4.6.1.2.1 in the NVMe 1.3 spec), indicates that
>>>> the command did not complete (As far as I can tell)
>>> My approach was exactly like that. In all cases other than WARN_HIGHECC we don't have a valid data. Without setting a bio_io_error() we are creating the impression for other layers, that we read the data correctly, what is not a case then.
>>> I'm also seeing that this patch is not the only user of bio_io_error() API, also other drivers such as md uses is commonly.
>>
>> Yes agree. This is an actual error in pblk that lets it return bogus data.
> 
> I am not against returning an error, I am just saying that this is not
> normal behavior on the read path.
> 
> The problem is that the upper layers might interpret that the device is
> broken completely, which is not true for a spa failing. Think for
> example of a host reading under mw_cunits - in reality this is not even
> a device problem but a host bug that might result in a fatal error.

Agree, and the host should manage it. The drive shall return a fatal 
error when it is asked to return invalid data. E.g., mw_cunits.

> 
> Matias: I am surprised to see you answer this way - when I tried to
> define a sane read error path with meaningful errors starting in the
> spec and all the way up the stack you were the first one to argue for
> reads to always succeed no matter what. In fact, using ZBC/ZAC as an
> example...

What I objected against was having error messages for each type of error 
that could happen. Instead of a single or few error types (that triggers 
the same set of recovery procedure at the host-side).