Date:   Tue, 3 Apr 2018 16:28:24 +0200
From:   Javier González <javier@...igon.com>
To:     Javier González <javier@...igon.com>
Cc:     Matias Bjørling <mb@...htnvm.io>,
        Jens Axboe <axboe@...nel.dk>, shli@...nel.org,
        linux-raid@...r.kernel.org, linux-block@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Huaicheng Li <huaicheng@...uchicago.edu>
Subject: Re: problem with bio handling on raid5 and pblk

> On 23 Mar 2018, at 13.52, Javier González <javier@...igon.com> wrote:
> 
>> On 22 Mar 2018, at 18.00, Matias Bjørling <mb@...htnvm.io> wrote:
>> 
>> On 03/22/2018 03:34 PM, Javier González wrote:
>>> Hi,
>>> I have been looking into a bug report when using pblk and raid5 on top
>>> and I am having problems understanding if the problem is in pblk's bio
>>> handling or on raid5's bio assumptions on the completion path.
>>> The problem occurs on the read path. In pblk, we take a reference to
>>> every read bio as it enters, and release it after completing the bio.
>>>   generic_make_request()
>>>   pblk_submit_read()
>>>     bio_get()
>>>     ...
>>>     bio_endio()
>>>     bio_put()
>>> The problem seems to be that on raid5's bi_end_io completion path,
>>> raid5_end_read_request(), bio_reset() is called. When put together
>>> with pblk's bio handling:
>>>   generic_make_request()
>>>   pblk_submit_read()
>>>     bio_get()
>>>     ...
>>>     bio_endio()
>>>     raid5_end_read_request()
>>>       bio_reset()
>>>     bio_put()
>>> it results in the newly reset bio being put immediately, thus freed.
>>> When the bio is reused then, we have an invalid pointer. In the report
>>> we received things crash at BUG_ON(bio->bi_next) at
>>> generic_make_request().
>>> As far as I understand, it is part of the bio normal operation for
>>> drivers under generic_make_request() to be able to take references and
>>> release them after bio completion. Thus, in this case, the assumption
>>> made by raid5, that it can issue a bio_reset() is incorrect. But I might
>>> be missing an implicit cross layer rule that we are violating in pblk.
>>> Any ideas?
>>> This said, after analyzing the problem from pblk's perspective, I see
>>> not reason to use bio_get()/bio_put() in the read path as it is at the
>>> pblk level that we are submitting bio_endio(), thus we cannot risk the
>>> bio being freed underneath us. Is this reasoning correct? I remember I
>>> introduced these at the time there was a bug on the aio path, which was
>>> not cleaning up correctly and could trigger an early bio free, but
>>> revisiting it now, it seems unnecessary.
>>> Thanks for the help!
>>> Javier
>> 
>> I think I sent a longer e-mail to you and Huaicheng about this a while back.
> 
> I don't think I was in that email.
> 
> There are two parts to the question. One is raid5's bio completion
> assumptions and the other is whether we can avoid bio_get()/put() in
> pblk's read path. The first part is pblk independent and I would like to
> leave it open as I would like to understand how bio_reset() in this
> context is correct. Right now, I cannot see how this is correct
> behaviour.
> 
> For the pblk specific part, see below.
> 
>> The problem is that the pblk encapsulates the bio in its own request.
>> So the bio's are freed before the struct request completion is done
>> (as you identify). If you can make the completion path (as bio's are
>> completed before the struct request completion fn is called) to not
>> use the bio, then the bio_get/put code can be removed.
>> 
>> If it needs the bio on the completion path (e.g., for partial reads,
>> and if needed in the struct request completion path), one should clone
>> the bio, submit, and complete the original bio afterwards.
> 
> I don't follow how the relationship with struct request completion is
> different with bio_get/put and without.
> 
> The flow in terms of bio and struct request management is today:
> 
>  generic_make_request()
>   pblk_submit_read()
>     bio_get()
>      ...
>     blk_init_request_from_bio()
>     blk_execute_rq_nowait() / blk_execute_rq() // denepnding on sync/async
>      ...
>     bio_endio()
>     bio_put()
>      ...
>     blk_mq_free_request()
> 
> bios risk to always freed in any case, as bio_put() will the last pblk
> reference. The only case in which this will not happen is that somebody
> else took a bio_get() on the way down. But we cannot assume anything.
> 
> I guess the problem I am having understanding this is how we can risk
> the bio disappearing underneath when we are the ones completing the bio.
> As I understand it, in this case we are always guaranteed that the
> bio is alive due to the allocation reference. Therefore, bio_get()/put()
> is not needed. Am I missing anything?
> 
> Thanks,
> Javier

After an offline discussion with Matias, we have agreed that
bio_get/put() should not be necessary on pblk's read path (Matias,
please correct me if my understanding is not correct). As mentioned
before too, we have been running internally without them for some time
without problems. I'll send a patch for the next window removing them.

This said, I believe that a generic_make_request driver should be able
to do bio_get/put without side consequences. Thus, I still doubt that
raid5's assumption that it can reset the bio on bio_end_io is correct
(and if yes, I would like to understand why). Thoughts?

Javier


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