linux-kernel - Re: [GIT pull] printk updates for 4.15

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Date:   Tue, 14 Nov 2017 15:18:47 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Mark Salyzyn <salyzyn@...roid.com>, Petr Mladek <pmladek@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [GIT pull] printk updates for 4.15

On Tue, Nov 14, 2017 at 3:04 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> True. I already replaced all of them except this one:
>
> @@ -1022,6 +1026,7 @@ void log_buf_vmcoreinfo_setup(void)
>          */
>         VMCOREINFO_STRUCT_SIZE(printk_log);
>         VMCOREINFO_OFFSET(printk_log, ts_nsec);
> +       VMCOREINFO_OFFSET(printk_log, ts);
>         VMCOREINFO_OFFSET(printk_log, len);
>         VMCOREINFO_OFFSET(printk_log, text_len);
>         VMCOREINFO_OFFSET(printk_log, dict_len);
>
> If I remove ts_nsec completely, then the VMCORE info changes and that would
> break existing tools because they search for ts_nsec and fail....

Dear god, that's nasty.

I agree that the vmcore info name probably needs to be retained, but
maybe we could just do that with

#define VMCOREINFO_FIELD_AND_OFFSET(name, field, offset) \
        vmcoreinfo_append_str("OFFSET(%s.%s)=%lu\n", name, field, offset)

and then do

  #define VMCOREINFO_OFFSET(name, field) \
        VMCOREINFO_FIELD_AND_OFFSET(#name, #field, \
                (unsigned long)offsetof(struct name, offset))

and for that ts_nsec you can then do

   /* ts.mono used to be called "ts_nsec" */
   VMCOREINFO_FIELD_AND_OFFSET("printk_log", "ts_nsec",
        (unsigned long)offsetof(struct printk_log, ts.mono))

or something.

And I may obviously have messed that all up, I wrote it all in the
email client, so it's more pseudo-code than anything else.

But it looks pretty straight-forward conceptually, and would make it
pretty clear what is going on, no?

              Linus