Re: MADV_DONTNEED

[James Antill <james@and.org>]

> Chuck Lever wrote:
> > > If I knew what msync(MS_INVALIDATE) did I could think about this! :-)
> > > But the msync documentation is unhelpful and possibly misleading.
> > 
> > well, the doc's accurate, as far as i can tell.  but my use of it is a
> > side-effect of the behavior described in the man page.
> 
> 	"MS_INVALIDATE asks to invalidate  other  mappings  of  the
>        same file (so that they can be updated with the fresh val-
>        ues just written)."
> 
> Oh I see.  It means the locally modified but in principle shared mapping
> is copied back to the underlying object.  For a page aligned mapping
> that shouldn't need to do anything.
> 
> Since the MS_INVALIDATE code doesn't modify other ptes, we must assume
> the other mappings are all page aligned or they wouldn't see the
> update.
> 
> So why does MS_INVALIDATE have any code? :-)

 I've used this in Solaris when mmap()'ing over NFS.

 Ie. You'd msync(MS_SYNC) on the NFS writer, and msync(MS_INVALIDATE)
on the readers.

 The Linux documentation I have is the same as Jamie's and says
_other_ mappings, but maybe that's just a typo (I'm pretty sure
INVALIDATE on solaris guaranteed that your mapping was invalidas
well).

-- 
James Antill -- james@and.org
"If we can't keep this sort of thing out of the kernel, we might as well
pack it up and go run Solaris." -- Larry McVoy.
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