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

[Suleiman Souhlal <ssouhlal@freebsd.org>]

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.

-- Suleiman
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