Re: XIP

[Masao Uebayashi <uebayasi%tombi.co.jp@localhost>]

Hi,

On Tue, Nov 02, 2010 at 08:58:14AM +0000, Mindaugas Rasiukevicius wrote:
> Chuck Silvers <chuq%chuq.com@localhost> wrote:
> > > >  - in getpages, rather than allocating a page of zeros for each file,
> > > >    it would be better to use the same page of zeroes that mem.c uses.
> > > >    if multiple subsystems need a page of zeroes, then I think that
> > > >    UVM should provide this functionality rather than duplicating
> > > >    that all over the place.  if you don't want to update all the
> > > > copies of mem.c, that's fine, but please put this in UVM instead of
> > > > genfs.
> > > 
> > > I made it per-vnode, because If you share a single physical zero'ed
> > > page, when a vnode is putpage'ed, it invalidates all mappings in
> > > other vnodes pointing to the zero page in a pmap.  Doesn't this
> > > sound strange?
> > 
> > I think you're misunderstanding something.  for operations like
> > msync(), putpage is called on the object in the UVM map entry,
> > which would never be the object owning the global page of zeroes.
> > from the pagedaemon, putpage is called on a page's object,
> > but if the page replacement policy decides to reclaim the page
> > of zeroes then invalidating all the mappings is necessary.
> > if what you describe would actually happen, then yes,
> > I would find it strange.
> 
> FYI: In rmind-uvmplock branch, all MD mem.c are replaced with a single MI
> driver, which provides zero page.  So that can be easily moved to UVM (and
> some day.. in the longer term, we should have a page for each NUMA node).

The difference is, mem.c and others use their zero page from kernel.
In XIP it's only for vnodes.  I'm not sure how much per-CPU zero
page helps XIP.  Anyway, I think my per-vnode approach is not
terribly bad.  Let's improve this later.

BTW, can't you merge your mem.c changes first?

Masao

> 
> > > Another reason is that by having a vm_page with a uvm_object as
> > > its parent, fault handler doesn't need to know anything about the
> > > special zero'ed page.
> > 
> > the fault handler wouldn't need to know about the page of zeroes,
> > why do you think it would?  it does need to be aware that the pages
> > returned by getpages may not be owned by the object that getpages
> > is called with, but it needs to handle that regardless, for layered
> > file systems.
> 
> Lock sharing among UVM objects (for layered file systems and tmpfs) should
> bring some simplifications here, I think.
> 
> -- 
> Mindaugas

-- 
Masao Uebayashi / Tombi Inc. / Tel: +81-90-9141-4635