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SubjectRe: [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

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