Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> On Fri, 13 Apr 2007 10:15:24 +1000 Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> > > > +               for (; i < 2 * chunk / KPMSIZE; i += 2, pfn++) {
> > > > +                       ppage = pfn_to_page(pfn);
> > > > +                       if (!ppage) {
> > > > +                               page[i] = 0;
> > > > +                               page[i + 1] = 0;
> > > > +                       } else {
> > > > +                               page[i] = ppage->flags;
> > > > +                               page[i + 1] = atomic_read(&ppage->_count);
> > > > +                       }
> > > > +               }
> > >
> > >
> > > Not a good idea to expose raw flags in this manner - it changes at the drop
> > > of a hat.  We'd need to also expose the kernel's PG_foo-to-bitnumber
> > > mapping to make this viable.
> >
> > I don't think it is viable because that makes the flags part of the
> > userspace ABI.
>
>
> It *will* be viable.  If the application wants to know if a page is dirty,
> it looks up "PG_dirty" in /proc/pg_foo-to-bitnumber and uses PG_dirty's
> numerical offset when inspecting fields in /proc/kpagemap.  If correctly
> designed, such a monitoring application will be able to report upon page
> flags which we haven't even thought up yet.

Ooh, you wanted a _runtime_ mapping of flags, yeah then I guess that works.
Still seems like a basically hit and miss affair to just use flags. What if
you want to know the process mapping a page? With systemtap or something you
could walk the rmap structures. What if you want to look at pages along the
LRU list rather than per-pfn? What about connecting pages to inodes?

I thought this type of deep poking was the whole reason the probles thingies
were merged. I'm saddened that they're no good for this. I thought it would
be an ideal usage :(


> > I wonder what they are needed for.
>
>
> Poking deeply into the kernel to provide information about kernel state. 
>
> There are real-world needs for this, and the people who develop tools to
> process this information will have decent kernel understanding and will
> know that the file's contents may alter across kernel versions.  It sure
> beats poking around in /dev/kmem.
>
> I doubt if there's a sensible way in which we can prettify this interface
> without losing information.  But we should aim to make it as robust as
> possible agaisnt future kenrel changes, of course.
>
> And we should satisfy ourselves that all the required information has been
> made available.  The fact that it will satisfy the Oracle requirement is
> encouraging.

Yeah it is close, they need page_mapcount I think. But I was going to say
that satisfying an Oracle requirement is a good reason _not_ to merge it ;)
(I joke!)

--
SUSE Labs, Novell Inc.
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