linux-kernel - Re: [PATCH v6 00/12] extend task comm from 16 to 24

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Message-ID: <YXfaA2uSj9JIfZIl@alley>
Date:   Tue, 26 Oct 2021 12:35:47 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Yafang Shao <laoar.shao@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Valentin Schneider <valentin.schneider@....com>,
        "Zhang, Qiang" <qiang.zhang@...driver.com>, robdclark@...omium.org,
        Christian Brauner <christian@...uner.io>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        dennis.dalessandro@...nelisnetworks.com,
        mike.marciniszyn@...nelisnetworks.com,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, linux-rdma@...r.kernel.org,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, linux-perf-users@...r.kernel.org,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <oliver.sang@...el.com>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH v6 00/12] extend task comm from 16 to 24

On Mon 2021-10-25 17:05:03, Steven Rostedt wrote:
> On Mon, 25 Oct 2021 11:10:09 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> > It looks like a churn that doesn't really address the problem.
> > If we were to allow long names then make it into a pointer and use 16 byte
> > as an optimized storage for short names. Any longer name would be a pointer.
> > In other words make it similar to dentry->d_iname.
> 
> That would be quite a bigger undertaking too, as it is assumed throughout
> the kernel that the task->comm is TASK_COMM_LEN and is nul terminated. And
> most locations that save the comm simply use a fixed size string of
> TASK_COMM_LEN. Not saying its not feasible, but it would require a lot more
> analysis of the impact by changing such a fundamental part of task struct
> from a static to something requiring allocation.

I fully agree. The evolution of this patchset clearly shows how many
code paths depend on the existing behavior.


> Unless you are suggesting that we truncate like normal the 16 byte names
> (to a max of 15 characters), and add a way to hold the entire name for
> those locations that understand it.

Yup. If the problem is only with kthreads, it might be possible to
store the pointer into "struct kthread" and update proc_task_name().
It would generalize the solution already used by workqueues.
I think that something like this was mentioned in the discussion
about v1.

Best Regards,
Petr