Re: [PATCH] mm: avoid dirtying shared mappings on mlock

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2007-10-12 at 10:45 -0700, Suleiman Souhlal wrote:
> On Oct 12, 2007, at 7:58 AM, Peter Zijlstra wrote:
> 
> > On Fri, 2007-10-12 at 07:53 -0700, Arjan van de Ven wrote:
> >> On Fri, 12 Oct 2007 12:50:22 +0200
> >>>>> The pages will still be read-only due to dirty tracking, so the
> >>>>> first write will still do page_mkwrite().
> >>>>
> >>>> Which can SIGBUS, no?
> >>>
> >>> Sure, but that is no different than any other mmap'ed write. I'm not
> >>> seeing how an mlocked region is special here.
> >>>
> >>> I agree it would be nice if mmap'ed writes would have better error
> >>> reporting than SIGBUS, but such is life.
> >>
> >> well... there's another consideration
> >> people use mlock() in cases where they don't want to go to the
> >> filesystem for paging and stuff as well (think the various iscsi
> >> daemons and other things that get in trouble).. those kind of uses
> >> really use mlock to avoid
> >> 1) IO to the filesystem
> >> 2) Needing memory allocations for pagefault like things
> >> at least for the more "hidden" cases...
> >>
> >> prefaulting everything ready pretty much gives them that... letting
> >> things fault on demand... nicely breaks that.
> >
> > Non of that is changed. So I'm a little puzzled as to which side you
> > argue.
> 
> I think this might change the behavior in case you mlock sparse files.
> I guess currently the holes disappear when you mlock them, but with  
> the patch the blocks wouldn't get allocated until they get written to.

Sure, but by point 1 - avoiding IO - that doesn't matter. Once you write
to a shared mapping you'll generate IO and you'll hit kernel allocations
and other delays no matter what you do.