linux-kernel - Re: [PATCH] speed up /proc/pid/stat, statm

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Date:	Wed, 1 Feb 2012 08:55:46 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] speed up /proc/pid/stat, statm

On Tue, 31 Jan 2012 12:36:12 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 31 Jan 2012 17:14:43 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > +int seq_put_decimal_ll(struct seq_file *m, char delimiter,
> > +			long long num)
> > +{
> > +	if (num < 0) {
> > +		if (m->count + 3 >= m->size) {
> > +			m->count = m->size;
> 
> Why is m->count udpated even thought we didn't write any data? 
> seq_put_decimal_ull() does it too.
> 

seq_xxxx functions set m->count == m->size when it finds possible buffer overflow.
If m->count == m->size after ->show(), buffer will be freed and twice size buffer
will be re-allocated, and retry.

If we don't set m->count == m->size here, following seq_putc() will succeed and
we'll see corrupted outputs.


Thanks,
-Kame



> > +			return -1;
> > +		}
> > +		if (delimiter)
> > +			m->buf[m->count++] = delimiter;
> > +		num = -num;
> > +		delimiter = '-';
> > +	}
> > +	return seq_put_decimal_ull(m, delimiter, num);
> > +
> > +}
> > +EXPORT_SYMBOL(seq_put_decimal_ll);
> 

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