Subject: Re: [PATCH] mm: avoid dirtying shared mappings on mlock
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>,
       Suleiman Souhlal <ssouhlal@freebsd.org>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       Suleiman Souhlal <suleiman@google.com>, linux-mm <linux-mm@kvack.org>,
       hugh <hugh@veritas.com>
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Date: Fri, 12 Oct 2007 16:58:25 +0200
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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().
> > >=20
> > > Which can SIGBUS, no?
> >=20
> > Sure, but that is no different than any other mmap'ed write. I'm not
> > seeing how an mlocked region is special here.
> >=20
> > I agree it would be nice if mmap'ed writes would have better error
> > reporting than SIGBUS, but such is life.
>=20
> 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...
>=20
> 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.

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