linux-kernel - Re: [RFC V2] printk: add warning while drop partial text in msg

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Date:   Mon, 18 Sep 2017 18:39:05 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     pierre kuo <vichy.kuo@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] printk: add warning while drop partial text in msg

Hi,

On (09/12/17 14:07), pierre kuo wrote:
[..]
> int per_cpu_thread_fn(void* data)
> {
>     unsigned int index = 0;
>     unsigned int len = 0;
>     char* local_string = kzalloc(2048, GFP_KERNEL);
> 
>     do {
>         len += sprintf((local_string + len), "this is the %d line\n", index++);
>     }while(len < 576);
>     printk_deferred("%s", local_string);
>     return 0;
> }

which is not a real world example, isn't it?

printk_deferred("%s", local_string[2048]) makes no sense anyway,
since we limit the message size to 1024 - HEADER chars in
vprintk_emit()  // see static char textbuf[LOG_LINE_MAX].

I'm not quite following what were you trying to prove, sorry.
does any function in the upstream kernel printk()-s buffers
larger than LOG_LINE_MAX? which one?


- the longest line in my dmesg output is 260 bytes in total (including timestamp)
- the second longest is 237 bytes (including timestamp)
- the third largest is 191 bytes (including timestamp)


the longest single printk() line known to me is from OOM code:

		show_node(zone);
		printk(KERN_CONT
			"%s"
			" free:%lukB"
			" min:%lukB"
			" low:%lukB"
			" high:%lukB"
			" active_anon:%lukB"
			" inactive_anon:%lukB"
			" active_file:%lukB"
			" inactive_file:%lukB"
			" unevictable:%lukB"
			" writepending:%lukB"
			" present:%lukB"
			" managed:%lukB"
			" mlocked:%lukB"
			" kernel_stack:%lukB"
			" pagetables:%lukB"
			" bounce:%lukB"
			" free_pcp:%lukB"
			" local_pcp:%ukB"
			" free_cma:%lukB"
			"\n",
			zone->name,
			K(zone_page_state(zone, NR_FREE_PAGES)),
			K(min_wmark_pages(zone)),
			K(low_wmark_pages(zone)),
			K(high_wmark_pages(zone)),
			K(zone_page_state(zone, NR_ZONE_ACTIVE_ANON)),
			K(zone_page_state(zone, NR_ZONE_INACTIVE_ANON)),
			K(zone_page_state(zone, NR_ZONE_ACTIVE_FILE)),
			K(zone_page_state(zone, NR_ZONE_INACTIVE_FILE)),
			K(zone_page_state(zone, NR_ZONE_UNEVICTABLE)),
			K(zone_page_state(zone, NR_ZONE_WRITE_PENDING)),
			K(zone->present_pages),
			K(zone->managed_pages),
			K(zone_page_state(zone, NR_MLOCK)),
			zone_page_state(zone, NR_KERNEL_STACK_KB),
			K(zone_page_state(zone, NR_PAGETABLE)),
			K(zone_page_state(zone, NR_BOUNCE)),
			K(free_pcp),
			K(this_cpu_read(zone->pageset->pcp.count)),
			K(zone_page_state(zone, NR_FREE_CMA_PAGES)));

but I believe even this line should be less than 1K bytes.
or are you aware of any cases when it passes the 1k limit?

we are straggling to resolve the _existing_ printk issues, so
_theoretical_ and never seen problems are not on my radar.

	-ss