Subject: Re: [RFC PATCH bpf-next v2 4/4] error-injection: Support fault
 injection framework
To: Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
References: <151427438796.32561.4235654585430455286.stgit@devbox>
 <151427450538.32561.2776225740675148782.stgit@devbox>
 <20171227021255.sahkcpgyzohl5brs@ast-mbp>
 <20171227170910.5ac1074bc86341f194130119@kernel.org>
CC: Josef Bacik <jbacik@fb.com>, <rostedt@goodmis.org>, <mingo@redhat.com>,
        <davem@davemloft.net>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <ast@kernel.org>, <kernel-team@fb.com>,
        <daniel@iogearbox.net>, <linux-btrfs@vger.kernel.org>,
        <darrick.wong@oracle.com>, Josef Bacik <josef@toxicpanda.com>,
        Akinobu Mita <akinobu.mita@gmail.com>
From: Alexei Starovoitov <ast@fb.com>
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Date: Wed, 27 Dec 2017 14:49:46 -0800
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On 12/27/17 12:09 AM, Masami Hiramatsu wrote:
> On Tue, 26 Dec 2017 18:12:56 -0800
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>
>> On Tue, Dec 26, 2017 at 04:48:25PM +0900, Masami Hiramatsu wrote:
>>> Support in-kernel fault-injection framework via debugfs.
>>> This allows you to inject a conditional error to specified
>>> function using debugfs interfaces.
>>>
>>> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
>>> ---
>>>  Documentation/fault-injection/fault-injection.txt |    5 +
>>>  kernel/Makefile                                   |    1
>>>  kernel/fail_function.c                            |  169 +++++++++++++++++++++
>>>  lib/Kconfig.debug                                 |   10 +
>>>  4 files changed, 185 insertions(+)
>>>  create mode 100644 kernel/fail_function.c
>>>
>>> diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
>>> index 918972babcd8..6243a588dd71 100644
>>> --- a/Documentation/fault-injection/fault-injection.txt
>>> +++ b/Documentation/fault-injection/fault-injection.txt
>>> @@ -30,6 +30,11 @@ o fail_mmc_request
>>>    injects MMC data errors on devices permitted by setting
>>>    debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request
>>>
>>> +o fail_function
>>> +
>>> +  injects error return on specific functions by setting debugfs entries
>>> +  under /sys/kernel/debug/fail_function. No boot option supported.
>>
>> I like it.
>> Could you document it a bit better?
>
> Yes, I will do in next series.
>
>> In particular retval is configurable, but without an example no one
>> will be able to figure out how to use it.
>
> Ah, right. BTW, as I pointed in the covermail, should we store the
> expected error value range into the injectable list? e.g.
>
> ALLOW_ERROR_INJECTION(open_ctree, -1, -MAX_ERRNO)
>
> And provide APIs to check/get it.

I'm afraid such check would be too costly.
Right now we have only two functions marked but I expect hundreds more
will be added in the near future as soon as developers realize the
potential of such error injection.
All of ALLOW_ERROR_INJECTION marks add 8 byte overhead each to .data.
Multiple by 1k and we have 8k of data spent on marks.
If we add max/min range marks that doubles it for very little use.
I think marking function only is enough.