linux-kernel - RE: pstore/ramoops - why only collect a partial dmesg?

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Message-ID: <a361c64213e7474ea39c97f7f7bd26ec@intel.com>
Date:   Tue, 4 Jan 2022 17:00:57 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        "Guilherme G. Piccoli" <kernel@...ccoli.net>
Subject: RE: pstore/ramoops - why only collect a partial dmesg?

> Hi Tony, thanks a lot for your response! It makes sense indeed, but in
> my case, for example, I have a "log_buf_len=4M", but cannot allocate a
> 4M record_size - when I try that, I can only see page_alloc spews and
> pstore/ramoops doesn't work. So, I could allocate 2M and that works
> fine, but I then lose half of my dmesg heh
> Hence my question.
>
> If there's no special reason, I guess would make sense to allow ramoops
> to split the dmesg, what do you think?

Guilherme,

Linux is indeed somewhat reluctant to hand out allocations > 2MB. :-(

Do you really need the whole dmesg in the pstore dump?  The expectation
is that systems run normally for a while. During that time console logs are
saved off to /var/log/messages.

When the system crashes, the last part (the interesting bit!) of the console
log is lost.  The purpose of pstore is to save that last bit.

So while you could add code to ramoops to save multiple 2MB chunks, it
doesn't seem (to me) that it would add much value.

-Tony