linux-kernel - Re: [PATCH 1/3] selftests/zram: Remove obsolete max_comp

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Date:   Wed, 26 Jan 2022 11:37:20 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Petr Vorel <pvorel@...e.cz>
Cc:     "xuyang2018.jy@...itsu.com" <xuyang2018.jy@...itsu.com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "naresh.kamboju@...aro.org" <naresh.kamboju@...aro.org>,
        "aleksei.kodanev@...l-sw.com" <aleksei.kodanev@...l-sw.com>,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 1/3] selftests/zram: Remove obsolete max_comp_streams
 interface

On 1/26/22 11:24 AM, Petr Vorel wrote:
>> On 1/26/22 12:13 AM, Petr Vorel wrote:
>>> Hi all,
> 
>>>> on 2022/1/26 4:33, Shuah Khan wrote :
>>>>> On 12/15/21 2:56 AM, Yang Xu wrote:
>>>>>> Since kernel commit 43209ea2d17a ("zram: remove max_comp_streams
>>>>>> internals"), zram has
>>>>>> switched to per-cpu streams. Even kernel still keep this interface for
>>>>>> some reasons, but
>>>>>> writing to max_comp_stream doesn't take any effect. So remove it.
> 
>>>>> I get that max_comp_stream doesn't do anything since this referenced
>>>>> commit. Don't we need this test on older kernels since older kernels
>>>>> still support max_comp_stream?
> 
>>>> I read the following info from kernel selftest documentation
>>>> https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
> 
>>>> "The kernel contains a set of “self tests” under the
>>>> tools/testing/selftests/ directory. These are intended to be small tests
>>>> to exercise individual code paths in the kernel. Tests are intended to
>>>> be run after building, installing and booting a kernel."
> 
>>>> So, we can build older kernel(use older kernel source) if we want to
>>>> test older kernel.
> 
>>>> IMO, kernel selftest is different from other testsuit(ie ltp, this
>>>> shuould think about api changes because ltp may test on different kernel).
>>> Yes, that's how I understand the difference with approach of in kselftest - the
>>> kernel tree testsuite and LTP - the out-of-tree testsuite.
> 
> 
>> Removing max_comp_stream test appears to be motivated by the fact it isn't
>> needed on newer kernels.
> 
>> Kselftest from mainline can be run on older stable kernels. This is a use-case
>> for a lot test rings. The idea is that when a new test gets added for older
>> code to regression test a bug, we should be able to run that test on an older
>> kernel. This is the reason why we don't remove code that can still test an older
>> kernel and make sure it skips gracefully.
> 
> Thanks for clarifying this approach. It might be worth of documenting it in
> dev-tools/kselftest.rst.
> 

I will send out a patch clarifying this.

thanks,
-- Shuah