Re: [PATCH] mm: Fix printk format strings

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 11 Oct 2007 09:04:57 +0200 "Vegard Nossum" <vegard.nossum@gmail.com> wrote:

> > > -     printk("Active:%lu inactive:%lu dirty:%lu writeback:%lu unstable:%lu\n"
> > > -             " free:%lu slab:%lu mapped:%lu pagetables:%lu bounce:%lu\n",
> > > +     printk("Active:%lu inactive:%lu dirty:%lu writeback:%lu unstable:%lu\n",
> > >               global_page_state(NR_ACTIVE),
> > >               global_page_state(NR_INACTIVE),
> > >               global_page_state(NR_FILE_DIRTY),
> > >               global_page_state(NR_WRITEBACK),
> > > -             global_page_state(NR_UNSTABLE_NFS),
> > > +             global_page_state(NR_UNSTABLE_NFS));
> > > +     printk(" free:%lu slab:%lu mapped:%lu pagetables:%lu bounce:%lu\n",
> > >               global_page_state(NR_FREE_PAGES),
> > >               global_page_state(NR_SLAB_RECLAIMABLE) +
> > >                       global_page_state(NR_SLAB_UNRECLAIMABLE),
> >
> > I don't understand the reason for this change.
> 
> I'm sorry :). It helps to make one line per call only, since this
> allows changing the printk internals for the better by reducing some
> complexity. I believe this is a good thing. I have a patch that
> changes printk, but it assumes that each format string only contains a
> single line. Is this a very bad assumption to make? Or maybe I should
> have sent that change first and made a reference to it?
> 
> But don't you also agree, on the grounds of principle, that a single
> line per call is better style?

Well we do multiple-lines-per-printk in rather a lot of places and it has
two advantages:

- less text size (I expect)

- the printk is "atomic" in that the logically-connected output lines
  won't get tangled up with an intervening printk from another CPU, or from
  an interrupt handler on this CPU.

Those are pretty modest advantages and I guess we could live without them
if we got a significant gain from doing so.  But it'd take quite some
effort hunting down all the callsites, and preventing new ones from
occuring.