linux-kernel - Re: syslog size unread: was: [PATCH v3 3/3] printk: use the lockless ringbuffer

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Date:   Mon, 29 Jun 2020 23:51:40 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: syslog size unread: was: [PATCH v3 3/3] printk: use the lockless ringbuffer

On 2020-06-25, Petr Mladek <pmladek@...e.com> wrote:
> On Thu 2020-06-18 16:55:19, John Ogness wrote:
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -1609,11 +1633,15 @@ int do_syslog(int type, char __user *buf, int len, int source)
>>  		break;
>>  	/* Number of chars in the log buffer */
>>  	case SYSLOG_ACTION_SIZE_UNREAD:
>> +		if (source != SYSLOG_FROM_PROC) {
>> +			text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
>> +			if (!text)
>> +				return -ENOMEM;
>
> The buffer is needed only to count lines to count the size of added
> prefixes. Could we use the new prb_read_valid_info() that allows to
> get the number of lines without actually reading the buffer?

Yes!

For the next version I introduce a macro to iterate just the meta data:

#define prb_for_each_info(from, rb, s, i, lc) \
for ((s) = from; prb_read_valid_info(rb, s, i, lc); (s) = (i)->seq + 1)

This can be used in all 3 locations where prb_count_lines() is used. In
all three places, there is no need to be copying the data. And for
SYSLOG_ACTION_SIZE_UNREAD, there is no need for the record and kmalloc'd
buffer.

This also means that prb_count_lines() will become a private static
helper of the ringbuffer.

John Ogness