linux-kernel - Re: [patch 2/2] Sort module list by pointer address to get coherent sleepable seq

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Date:	Wed, 15 Aug 2007 16:36:45 +0800
From:	Fengguang Wu <wfg@...l.ustc.edu.cn>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Randy Dunlap <rddunlap@...l.org>,
	Martin Bligh <mbligh@...gle.com>
Subject: Re: [patch 2/2] Sort module list by pointer address to get
	coherent sleepable seq_file iterators

On Wed, Aug 15, 2007 at 07:53:01AM +0100, Al Viro wrote:
> On Wed, Aug 15, 2007 at 02:37:41PM +0800, Fengguang Wu wrote:
> > static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
> > {
> >         loff_t *spos = (loff_t *) v;
> >         *pos = ++(*spos);
> >         return spos;
> > }
> > 
> > I mean 'pos' is sometimes increased in ct_seq_next(), and sometimes from
> > seq_file.c/seq_read(), too. Thus we cannot reliably do this:
> > 
> >         *pos = (*spos) + some_variable_offset;
> 
> Of course we can.  These guys can be sparse - note that ->start()
> takes a pointer, and for a good reason.  ->start(m, p, pos) should
> get the first entry with offset >= *pos (or NULL if we are done) and
> set *pos accordingly.
>
> That m->index++ is "we are done with the partial, step just past it, so
> that ->start() will pick the first real entry after it the next time it's
> called".
> 
> For dense case we don't need to update *pos in ->start() - either
> we already have one with offset == *pos (and no update is needed),
> or we are finished and should return NULL.
> 
> However, we have every right to live with sparse offsets; prototype of
> ->start() had been done the way it's done exactly to allow that kind
> of use.

So sparse offsets are supported, with some special cares on ->start.

My case is to scan the address space in ranges. The "object" is the
start offset of a range:

__________________#######______________________#############__________
                  ^start                       ^start

Now the solution can be:
        - ->show shows the current range
        - ->next seeks to next range
        - ->start must *also* do the seek

The last requirement is made clear by you, a fact I refused to accept :)
My old concept was that a ->next should be called to move pages
forward after a new start.

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