Re: ubifs: assertion fails

[Tanya Brokhman <tlinder@codeaurora.org>]

Hi Artem

On 3/31/2014 1:14 PM, Artem Bityutskiy wrote:
> On Mon, 2014-03-24 at 06:03 +0000, Dolev Raviv wrote:
>> Hi all,
>>
>> I’m doing my first steps learning ubifs and I’m trying to understand a
>> something that does not make much sense to me.
>>
>> In fs/ubifs/shrinker.c, at shrink_tnc(), there is an assert condition that
>> shows up every once I a while (after stressing).
>> ubifs_assert(atomic_long_read(&c->clean_zn_cnt) >= 0);
>
> When this happens, do you then see a storm of similar assertions from
> other parts of the code? I am trying to understand if this assertion is
> incorrect, or you really get the accounting screwed when shrinking
> happens.
>
> In the former case, this would probably be a single assertion, on the
> latter you'd probably see many similar warnings from other code. E.g.,
> when you unmount.
>

The log isn't flooded with the above mentioned error, but it does repeat 
several times.

>
>> Could the assertion condition be wrong?
>
> Could be, but could also show that there is an accounting error
> happening when shrinker starts.
>
> And I saw misterious errors when shrinker starts working at some point,
> but did not have time to dig this. So there is at least 1 bug in the
> shrinker path which I saw.

Is there any way we can help in debugging and fixing this? Also, we're 
running on a 3.10 based kernel and I saw a lot of patches that change 
the shrinker after 3.10 on linux-next. What kernel version did you see 
the shrinker errors on?

>
>> Can anyone share information on what are those times that the counter can
>> be negative?
>
> When the commit operation starts, it grabs the tnc_mutex, prepares the
> list of nodes to commit, and release tnc_mutex. Now the accounting is
> incorrect. When the commit finishes, it grabs the mutex again, does some
> stuff, and also fixes the accounting. Then drops the mutex.
>
> The idea was to make sure that commit does not block I/O. Meaning that
> you can still write files while commit is going on.
>

Thanks,
Tanya Brokhman
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